Causes of death were studied in 10,552 Swedish hyperthyroid patients treated with 131I diagnosed between 1950 and 1975. The patients were followed for an average of 15 years (range 0-35 years) and were matched with the Swedish Cause of Death Register. A total of 5,400 deaths were observed and the overall standardized mortality ratio was 1.47 (95% confidence interval (CI) 1.43-1.51). The standardized mortality ratio for females was 1.50 (95% CI 1.46-1.55) compared to 1.31 (95% CI 1.23-1.39) in males. The most common cause of death (61%) was from cardiovascular diseases (standardized mortality ratio 1.65; 95% CI 1.59-1.71). Significantly elevated risks were also seen for tumours, diseases of the endocrine system, respiratory system, gastro-intestinal system, and congenital malformations. In all causes of death, except tumours and trauma, decreasing standardized mortality ratios over time were seen. Patients followed for more than 10 years had significantly elevated risks for tumours, diseases of the endocrine, respiratory, and cardiovascular systems. Patients given higher 131I activity and younger patients had higher standardized mortality ratios than those given lower activity and older patients. The hyperthyroidism per se, rather than the 131I treatment, appeared to be the major explanation for the elevated mortality.