The Young Hearts (YH) Project is an ongoing study of biological and behavioural risk factors for cardiovascular disease in a representative sample of young people from Northern Ireland, a region of high coronary mortality. This article describes the cross-sectional clinical, dietary and lifestyle data obtained from individuals (aged 20-25 y) who participated in phase 3 of the project (YH3). A total of 489 individuals (251 males, 238 females) participated in YH3 (48.2% response rate). Some 31.1 % of participants at YH3 were overweight (BMI > 25 kg/m(2)) with 4.4% of males and 8.0% of females were obese (BMI > 30 kg/m(2)). More females than males had a very poor fitness (55.0 vs 22.1%, chi-squared 51.70, d.f. 1, P < 0.001) and did not participate in any sporting or exercise activity (38.4 vs 24.9%, chi-squared 10.26, d.f. 1, P = 0.001). Over 20% of participants had a raised total serum cholesterol (>5.2 mmol/l). More females had a raised serum LDL-cholesterol (>3.0 mmol/l) than males (44.6 vs 34.6%, chi-squared 4.39, d.f 1, P < 0.05). Over 46% of participants reported energy intakes from fat above recommended levels, and 68.5% of participants had saturated fat intakes above those recommended. (Dietary reference values for food energy and nutrients for the United Kingdom. HMSO: London, 199 1). Just over half of the study population reported alcohol intakes in excess of recommended sensible limits set by the Royal College of Physicians (A great and growing evil: the medical consequences of alcohol abuse. Tavistock: London, 1987), with 36.7% of males and 13.4% of females reporting intakes over twice these recommended limits. A total of 37% of the study population smoked. During young adulthood, individuals may be less amenable to attend a health-related study and recruitment of participants to the current phase of the study proved a major problem. However, these data constitute a unique developmental record from adolescence to young adulthood in a cohort from Northern Ireland and provide additional information on the impact of early life, childhood and young adulthood on the development of risk for chronic disease.