This prospective study of 1,561 patients with leukemia, 1,871 with lymphosarcomas, 1,028 with Hodgkin's disease, and 207 with myeloma revealed the following: In all, 95 further cancers were found during the 9,840 patient-years of observation when only 45.4 were expected from New York State incidence rates for a population of this sex and age distribution. Prostate, bladder, endometrial, and breast cancers were somewhat more frequent than expected. The four patients with Kaposi's sarcoma only confirm the association of this tumor with the index diseases. The greatest increase was in skin cancers: 6 were expected, whereas 43 patients developed over 60 asynchronous carcinomas. This increase is significant, even allowing for a likely 2½-fold underregistration in the control tabulation. The minimum increases were 9-fold after myeloid leukemia, 4-fold after lymphatic leukemia, 2½-fold after lymphosarcoma, and 2-fold after Hodgkin's disease. Squamous carcinomas, basal cell carcinomas, and melanomas all had the same degree of increase. No single common cause for this increase was found, though a number of patients reported specific exposures to carcinogenic situations.