INTRODUCTION: Lithium treatment is claimed to reduce mortality in patients with affective disorder, but the evidence is conflicting. The aim of this study was to estimate mortality rates from a cohort of such patients, who commenced treatment with lithium, over an observation period of 16 years. MATERIAL AND METHODS: The mortality rates of affectively ill patients, who commenced lithium treatment, were compared with the mortality rates in the general Danish population, standardised for age, sex, and a time to death from all causes, suicide, and cardiovascular death. Comparison of the time from a two-year follow-up to death from any cause between patients compliant and non-compliant with two years of lithium treatment was performed with the Cox regression analysis. RESULTS: Forty of the 133 patients who participated in the study died during the 16-year observational period; 11 from suicide. Mortality in the patients was twice that of the background population. This statistically significantly higher mortality was predominantly caused by the number of suicides, whereas mortality from all other causes was similar to the background population. Thirty-two patients died after the first two years of observation and were included in the analysis of association between death and two years of compliance with lithium. Suicide occurred more frequently in the lithium non-compliant patients than in the lithium compliant patients. DISCUSSION: Mortality, particularly death from suicide, was significantly increased in unselected affective disorder patients, who commenced lithium treatment, as compared with the background population.