Eight children treated between 1977 and 1983 with the UK Children's Cancer Study Group's non-Hodgkin lymphoma (NHL) and T-cell protocols have developed second malignancies within 7 years of commencing treatment. Five developed acute non-lymphoblastic leukaemia and a sixth died from infection while pancytopenic with a pre-leukaemic marrow. The other malignancies were cerebral astrocytoma and an undifferentiated low grade sarcoma. These eight children were included among 261 children studied in the first UKCCSG NHL and T-cell trials giving an actuarial incidence of 7.8% second malignancy at 7 years. Six had received adjuvant radiotherapy which may have contributed to the high incidence of second malignancy.