In 1976, an accident in a plant near Seveso, Italy, exposed the local population to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Persons residing in three zones of decreasing TCDD contamination (A, B, and R) and a reference population were followed up for cancer occurrence in 1977-1986. The most exposed subgroup (A) was small, and only 14 cancer cases were observed. In zone B, hepatobiliary cancer was elevated, especially for those living in the area for > 5 years [relative risk (RR) = 2.8; 95% confidence interval (CI) = 1.2-6.3]. Men exhibited an increase in hematologic neoplasms, most notably lymphoreticulosarcoma (RR = 5.7; 95% CI = 1.7-19.0). Women experienced an increased incidence of multiple myeloma (RR = 5.3; 95% CI = 1.2-22.6) and myeloid leukemia (RR = 3.7; 95% CI = 0.9-15.7). In zone R, the incidence of soft tissue tumors and non-Hodgkin's lymphomas was elevated, particularly among persons living in the area for > 5 years (RR = 3.5; 95% CI = 1.2-10.4 for sarcomas, and RR = 2.0; 95% CI = 1.2-3.6 for non-Hodgkin's lymphomas). Breast cancer among females was below expectations in the most contaminated zones, and a clear deficit for endometrial cancer was observed in zones B and R.