Thorotrast is a radioactive radiological contrast medium used in the 1920-50s. Thorotrast is retained by the reticuloendothelial system with a biological half-life of several hundred years, so that patients administered this contrast medium systemically suffer lifetime exposure to internal alpha-particle radiation. We studied mortality in Portuguese patients who were administered thorotrast and in a comparison group of patients who received non-radioactive contrast agents. A cohort of 1096 systemically-exposed thorotrast patients and 1014 unexposed patients was followed-up to the end of 1996. Mortality was statistically significantly raised among systemically-exposed thorotrast patients relative to those unexposed for all causes, liver cancer, chronic liver disease, other non-neoplastic diseases of the digestive system, neoplastic and non-neoplastic haematological disorders, and non-neoplastic diseases of the respiratory system. Risks for most of these conditions increased significantly with time since first administration remaining high for over 40 years after administration of thorotrast.