The risk of second malignancies following non-Hodgkin's lymphoma (NHL) was estimated in 29,153 patients diagnosed with NHL between 1973 and 1987 in one of nine areas participating in the National Cancer Institute's Surveillance, Epidemiology, and End Results Program. Compared with the general population, NHL patients were at a significantly increased risk of developing second cancers (observed/expected [O/E] = 1.18; O = 1231). The O/E ratio increased significantly with time to reach 1.77 in 10-year survivors. Significant excesses were noted for acute nonlymphocytic leukemia (O/E = 2.88), cancers of the bladder (O/E = 1.30), kidney (O/E = 1.47), and lung (O/E = 1.57), malignant melanoma (O/E = 2.44), and Hodgkin's disease (O/E = 4.16). Chemotherapy appeared related to subsequent acute nonlymphocytic leukemia (ANLL) and bladder cancer. Radiation therapy was associated with ANLL and possibly cancers of the lung, bladder, and bone. Malignant melanoma was not clearly related to initial NHL treatment.