The mortality experience of 19,637 people aged 1-19 years living in an area around Seveso, Italy, contaminated by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) after an accidental explosion in a chemical plant was examined for the period 1976-1986. For comparison, the mortality of the population of nearly 100,000 people living in the surrounding districts was examined. People who left the study area were followed up. Vital status ascertainment was successful in over 99% of the cases. A group of 186 children who contracted chloracne, a reversible marker of TCDD intoxication, shortly after the accident were in the study: none of them died during the observation period. Among the exposed, mortality owing to all causes, to all accidents and to all cancers failed to show major departure from expectations. Five leukaemia deaths were observed, four among males and one among females; the corresponding relative risks were 2.1 (95% confidence interval (CI): 0.7-6.9), and 2.5 (95% CI: 0.2-27.0), respectively. Two lymphatic leukaemias among males yielded a RR = 9.6 (95% CI: 0.9-106.0). Mortality owing to congenital anomalies showed a nearly twofold increase in the contaminated area; however, five out of seven observed cases were born before the accident. Interpretation is limited by the small number of deaths, short latency period and low validity of death certificate information for certain causes, and by the definition of exposure which was based merely on residence. The follow-up is continuing. A cancer morbidity study is in progress.