From 1965 to 1986, thirty-four patients with retinoblastoma were born in Nagasaki Prefecture, Japan, 23 with unilateral and 11 with bilateral retinoblastoma. The incidence of retinoblastoma was calculated to be approximately 1 in 16,053 births; it was bilateral in 1 in 49,618 births. There was no significant difference in the incidence of retinoblastoma between urban and rural areas. In Shimabara district it was 1 in 10,331 births--much higher than in any other district of Nagasaki Prefecture. Historically, almost all the natives had been killed in the Shimabara War in 1637, and then people immigrated to Shimabara mainly from Hyogo, Aichi and Shizuoka Prefectures. The authors compared the incidence and distribution of retinoblastoma in these three prefectures with the figures in Shimabara. The incidence in the Tamba and Tajima districts of Hyogo Prefecture was 1 in 10,570 and 1 in 10,411, respectively, very similar to that in Shimabara. The authors wonder if this similarity is only an accidental coincidence when the historical relationship among the three districts is considered.