OBJECTIVE: Elevated serum total homocysteine (tHcy) is considered an independent risk factor for cardiovascular disease. The objective of this study was to develop the first-ever information on the prevalence of hyperhomocysteinemia and its determinants in a population in Costa Rica. METHODS: A cross-sectional study was conducted to determine serum levels of tHcy, vitamin B(12), folate, and creatinine, as well as the presence of the genotype TT for the methylenetetrahydrofolate reductase (MTHFR) enzyme. Additionally, dietary vitamin intakes and other lifestyle risk factors were assessed. A total of 399 Costa Rican adults from the central valley of the country (where the capital city, San José, is located), aged 20 to 40 years, participated in this study in the year 2000. Analyses of variance were performed for continuous variables, and the chi-square test was used for categorical data. Spearman correlation tests were calculated to determine associations between variables. Three linear regression analyses and one binary logistic model were developed in order to determine the predictors for homocysteine levels in the population studied. RESULTS: The overall prevalence of hyperhomocysteinemia (>15 micromol/L) in the population was 6%, 31% of the population were in the range of 10 to 15 micromol/L, 29% had the genotype TT for the enzyme MTHFR, 18% presented a vitamin B(12) deficiency (<165 pmol/L), and none of the persons had low serum folate levels (<7.0 nmol/L). No significant associations were found between tHcy and age, smoking, consuming alcohol, or dietary vitamin intake. CONCLUSIONS: Only serum vitamin B(12) levels and the genotype TT of the enzyme MTHFR were considered significant predictors of high serum tHcy levels in the Costa Rica population studied.