BACKGROUND: By improving two social determinants of health (poverty and unemployment) in low- and middle-income families on or at risk of welfare, in-work tax credit for families (IWTC) interventions could impact health status and outcomes in adults. OBJECTIVES: To assess the effects of IWTCs on health outcomes in working-age adults (18 to 64 years). SEARCH METHODS: We searched 16 electronic academic databases, including the Cochrane Public Health Group Specialised Register, Cochrane Database of Systematic Reviews (The Cochrane Library 2012, Issue 7), MEDLINE and EMBASE, as well as six grey literature databases between July and September 2012 for records published between January 1980 and July 2012. We also searched key organisational websites, handsearched reference lists of included records and relevant journals, and contacted academic experts. SELECTION CRITERIA: We included randomised and quasi-randomised controlled trials and cohort, controlled before-and-after (CBA) and interrupted time series (ITS) studies of IWTCs in working-age adults. Included primary outcomes were: self rated general health; mental health/psychological distress; mental illness; overweight/obesity; alcohol use and tobacco use. DATA COLLECTION AND ANALYSIS: Two review authors independently extracted data and assessed the risk of bias in included studies. We contacted study authors to obtain missing information. MAIN RESULTS: Five studies (one CBA and four ITS) comprising a total of 5,677,383 participants (all women) fulfilled the inclusion criteria and were synthesised narratively. The in-work tax credit intervention assessed in all included studies is the permanent Earned Income Tax Credit in the United States, established in 1975. This intervention distributed nearly USD 62 billion to over 27 million individuals in 2011, and its administration costs were less than one per cent of its total costs. All included studies carried a high risk of bias (especially from confounding and insufficient control for underlying time trends). Due to the small number of (observational) studies and their high risk of bias, we judged this body of evidence to have very low overall quality.One study found that IWTC had no detectable effect on self rated general health and mental health/psychological distress five years after its implementation (i.e. a considerable change in the generosity of the permanent IWTC) and on overweight/obesity eight years after implementation. One study found no effect of IWTC on tobacco use five years after implementation, one a moderate reduction in tobacco use one year after implementation (odds ratio 0.95, 95% confidence interval (CI) 0.94 to 0.96), and one differential effects, with no effect in African-Americans and a large reduction in European-Americans two years after implementation (risk difference -11.1%, 95% CI -20.9% to -1.3%). No evidence was available for the effect of IWTC on mental illness and alcohol use. No adverse effects of IWTC were identified.One study also found no detectable effect of IWTC on the number of bad physical health days and of risky biomarkers for inflammation, cardiovascular disease and metabolic conditions eight years after implementation. One study found that IWTC had a large, positive effect on income from wages or salaries one year after implementation. Two studies found no effect on employment two and five years after implementation, whereas two found a moderate increase five and eight years after implementation and one a large increase in employment due to IWTC one year after implementation.No differences in outcomes between groups with different educational status were found for self rated health and mental health/psychological distress. In one study European-American women with lower levels of education were more likely to reduce tobacco use, while tobacco use did not change among African-American women with lower levels of education. However, no differences in tobacco use by educational status were observed in a second study. Two studies found that the intervention may have reduced inequity with respect to employment, where women with less education were more likely to move into employment (although one did not establish whether this difference was statistically significant), while two studies found no such difference and no studies found differences by ethnic group on employment rates. AUTHORS' CONCLUSIONS: In summary, the small and methodologically limited existing body of evidence with a high risk of bias provides no evidence for an effect of in-work tax credit for families interventions on health status (except for mixed evidence for tobacco smoking) in adults.