PURPOSE: Heart diseases are often associated with residual injuries, persisting functional restrictions, and long-term sequelae for psychosocial development. Currently, there are no disease-specific instruments to assess the health-related quality of life (HrQoL) of pre-school children. The aims of this study were to develop a parent proxy instrument to measure the HrQoL of children aged 3-7 years with a heart disease and to confirm its validity and reliability. METHODS: Items from the Preschool Pediatric Cardiac Quality of Life Inventory (P-PCQLI) were generated through focus groups of caregivers. In a pilot study, comprehensibility and feasibility were tested. Five subdimensions were defined theoretically. Psychometric properties were analysed within a multicentre study with 167 parental caregivers. RESULTS: The final 52-item instrument contains a total score covering five moderately inter-correlated dimensions. The total score of the questionnaire showed a very high internal consistency (Cronbachs' α = 0.95). Test-retest correlation was at r tt = 0.96. External validity was indicated by higher correlations (r = 0.24-0.68) with a generic paediatric quality of life questionnaire (KINDL) compared to the Strengths and Difficulties Questionnaire (r = 0.17 to 0.59). Low P-PCQLI total scores were significantly associated with inpatient as opposed to outpatient treatment (t = 6.04, p < .001), with at least moderate disease severity ((t = 5.05, p < .001) NYHA classification) and with poorer prognosis (t = 5.53, p < .001) as estimated by the physician. CONCLUSIONS: The P-PCQLI is reliable and valid for pre-school children with a heart disease. It could be used as a screening instrument in routine care, and for evaluation of HrQoL outcomes in clinical trials and intervention research.