The objective of this study was to assess the long-term outcome of polymyositis (PM) and dermatomyositis (DM) and the factors predictive of this outcome in a nationwide series in Finland. One hundred and seventy-six patients with PM and 72 patients with DM diagnosed in Finland in 1969-1985 were selected from the national hospital discharge register according to the diagnostic criteria of Bohan and Peter and followed up until death or till the end of August 1995. Gender, age, delay of therapy, serum creatine kinase, erythrocyte sedimentation rate, initial dose of corticosteroids and duration of cytostatic therapy were assessed as factors prognostic of death. The 5-year survival rate for PM was 75% [95% confidence interval (CI): 68-81%] and that for DM 63% (50-73%), and the respective 10-year survival rates were 55% (47-62%) and 53% (41-64%). The median survival for polymyositis was 11.0 years (95% CI: 9.5-13.3) and that for DM 12.3 years (5.5-20.7). The standardized mortality ratio for the combined group of PM and DM showed approximately threefold mortality compared to the general population. Compared to PM patients, the subjects with DM had a 1.47-fold (95% CI: 0.99-2.12) age- and sex-adjusted risk of mortality. Except for age in both groups and the delay in diagnosis in the PM group, no other individual factor reached significance as a predictor of death. However, cancer had a hazard ratio (HR) of 2.16 for death (95% CI: 0.95-4.50) in the DM group and 1.99 (95% CI: 1.01-3.94) in the PM group. A comparison of the causes of death in the PM and DM groups showed that the patients with DM had a greater risk of dying from cancer (age-adjusted HR 5.11, 95% CI: 2.31-11.3). According to this nationwide analysis of survival and its prognostic factors in patients with PM and DM, the latter group had an increased age-adjusted risk for mortality compared to the former. The difference seems to be mostly explained by the fact that the patients with DM had a higher risk of dying from cancer.