The hypothesis that a dietary Supplement of selenium (Se) may reduce cancer risk was tested experimetally in humans. Patients with histories of basal/squarnous cell carcinomas of the skin were assigned randomly in double-blind fashion to dally oral supplements of either Seenriched yeast (200 μg Se/day), or a low-Se yeast placebo. A total of 1312 patients recruited in 1983-1990 were followed with regular dermatologic examinations through 1993 for a total of 8269 person-years of observation. Skin cancer diagnoses were confirmed histologically.Plasma Se concentration was determined at 6-12 months intervals. All deaths and patient-reported illnesses were recorded; reported cancers were confirmed and documented by consultation with the patient medical care providers. The results indicate that Se did not significantly affect the primary endpoints: incidences of recurrent basal/squarnous cell carcinomas of the skin. However, Setreatment was associated with reductions in several secondary endpoints:total mortality, mortality from all cancers combined, as well as the incidence of all cancers combined, lung cancer, colorectal cancer and prostate cancer. The consistencies of these associations over time, between study clinics and for the leading cancer sltes strongly suggests benefits of Se-supplementation for this cohort of patients, supporting the hypopthesis that supplemental Se can reduce cancer risk. Although Se did no shown protective effects against nonmelanoma skin cancers, the suggested reductions in risks to other frequent cancers demand further evaluation in well controlled cliflical intervention trials The hypothesis that a dietary Supplement of selenium (Se) may reduce cancer risk was tested experimetally in humans. Patients with histories of basal/squarnous cell carcinomas of the skin were assigned randomly in double-blind fashion to dally oral supplements of either Seenriched yeast ( 200 μg Se/day), or a low-Se yeast placebo. A total of 1312 patients recruited in 1983-1990 were followed with regular dermatologic examinations through 1993 for a total of 8269 person-years of observation. Skin cancer diagnoses were confirmed histologically .Plasma Se concentration was determined at 6-12 months intervals. All deaths and patient-reported illnesses were recorded; reported cancers were confirmed and documented by consultation with the patient medical care providers. The results indicate that Se did not significantly affect the primary endpoints : incidences of recurrent basal/squarnous cell carcinomas of the skin. However, Setreatment was associated with reductions in several secondary en Donss:total mortality, mortality from all cancers combined, as well as the incidence of all cancers combined, lung cancer, colorectal cancer and prostate cancer. The consistencies of these associations over time, between study clinics and for the leading cancer sl but strictly culture benefits Of Se-supplementation for this cohort of patients, supporting the hypopthesis that can reduce reduce cancer risk. although Se did no illustrated protective effects against nonmelanoma skin cancers, the proposed reductions in risks to other frequent cancers demand further more evaluation in well controlled cliflical intervention Trials