OBJECTIVE: The study aimed to estimate the prevalence of mental disorders based on pharmaceutical use among the old (age >/= 65) in Sweden for the years 2006-2008. METHODS: Data on the mental health of older persons were approximated on the basis of recommended prescriptions for pharmaceuticals, gathered from the Swedish Register on Prescribed Pharmaceuticals (SRPP). Each disorder (ICD-10, F20-F42, and F60-F61) was analyzed to identify associated recommended pharmaceuticals. Anatomical Therapeutic Chemical Classification codes were applied. The data covered 188 024 individuals who received 2 013 079 prescriptions for pharmaceuticals for mental disorders during a 3-year period. Persons with pharmaceuticals for dementia disorders were excluded from the calculations of the prevalence of mental disorders. RESULTS: The prevalence of mental disorders among the old in Sweden, measured on the basis of pharmaceutical use, was 6.6% in 2006, 2007, and 2008, respectively. Men constituted one-third of cases and women two-thirds. Prevalence was lowest in the age group 65-69 and increased subsequently with age. CONCLUSIONS: This fundamental register-based study included a great number of older persons and shows that mental disorders affect every fifteenth older person in Sweden. The prevalence of mental disorders increases with increasing age. The results highlight the extent of mental disorders among older persons, which is important to know when planning care for these patients. This study, by investigating a large population, provides a solid basis for general planning as well as for future mental disorder research. Copyright (c) 2010 John Wiley & Sons, Ltd.