In an area of Tanzania in which Burkitt's lymphoma (BL) is endemic, five families are described in which multiple cases of BL were found or BL occurred with other neoplasms. The patients include two brothers and one half-brother with BL, two brothers with BL, a woman with nasopharyngeal carcinoma (NPC) whose daughter had BL, a boy with BL whose sister developed chronic myelogenous leukaemia (CML), and a man with CML whose son developed BL. The two full-sib pairs with BL is significantly more than would be expected to have arisen by chance and the association of BL with NPC and CML among close relatives is remarkable in view of the rarity of the last two neoplasms in the study area. It is suggested that genetic factors may be important. It is suggested that genetic factors may be important in determining susceptibility to the three malignancies in this population, but the possibility that the clustering of cases within families may be due to local environmental factors cannot be excluded.