To estimate the annual incidence of adult T-cell leukemia/lymphoma (ATL) by district in Japan, a large-scale nationwide survey of ATL and of non-Hodgkin's lymphoma was performed in 1988. Questionnaires for the registration of ATL and of T-cell and non-T-cell lymphoma were distributed to the physicians in charge of this survey in 1,287 hospitals with 200 or more beds throughout Japan. From the positive rate of anti-HTLV-I antibody in adults, the annual incidence of ATL was estimated at 697, independently of the present survey. In fact, 657 cases (47% of the estimated number), newly diagnosed during the 2 years January 1986 to December 1987, were registered from 191 general hospitals throughout Japan. Major results obtained from the present survey are as follows: (1) among all ATL cases registered, 51% were from Kyushu and 29% were from metropolitan areas (Kanto, Chubu and Kinki) and most, but not all, patients with ATL in the metropolitan areas had come from the ATL-endemic areas and settled in the metropolitan areas; (2) the estimated annual incidence rates of ATL per million adults were 40.4 in males and 26.4 in females in Kyushu, the overall risk of ATL being 1.5 times as high in males as in females; (3) the age-specific incidence rate in Kyushu increased steeply with age until the age of 70, and then decreased markedly in both sexes; (4) the ratio of T-cell versus non-T-cell lymphomas was 2.9 in Kyushu but 0.5 in other districts of Japan, however, this difference regressed to the average for the whole of Japan if ATL cases were excluded; (5) 26.5% of patients with ATL had a family history of cancer, and among these, 14 (8.2%) were ATL, 21 (12.2%) were lymphoma and 17 (9.9%) were hematopoietic malignancies, the incidence of which was markedly higher than in the general population; (6) with regard to clinicopathological features of ATL, there were more advanced cases in south Kyushu than in other districts, however, these differences were not statistically significant. To clarify the chronological changes and geographical variations in the annual incidence of ATL in Japan, continuous systematic nationwide surveillance is necessary and further nation-wide studies are being prepared.