Background The aim of this study was to assess the incidence of sickness absence with psychiatric diagnoses from 1994–2000, and the distribution across gender, age groups, diagnostic groups and regions in a general population. Methods The population at risk was defined as all individuals aged 16–66 years who were entitled to sickness benefits in 1994, 1996, 1998 and 2000 (n = 2,282,761 in 2000). All individuals with a full-time disability pension were excluded. The study included approximately 77% of the Norwegian population aged 16–66 years. For each year, the study base started on 1 January and ended on 31 December. Individuals that were sick-listed for more than 14/16 consecutive days with a psychiatric diagnosis on their medical certificate were selected as cases. Included in this study were data for Norway, the capital city Oslo and five regions in the southeast of the country. Results Sickness absence with psychiatric diagnoses increased in all age groups, in women and men, and in all regions. At the national level, the cumulative incidence increased in women from 1.7% in 1994 to 4.6% in 2000, and in men from 0.8% in 1994 to 2.2% in 2000. The highest cumulative incidence was found in middle-aged women and men (30–59 years). Women had a higher incidence than men in all stratification groups. The cumulative incidences in 2000 varied between 4.6% to 5.6% in women in the different regions, and for men the corresponding figures were 2.1% to 3.2%. Throughout the four years studied, women in Oslo had more than twice as high incidence levels of sickness absence with alcohol and drug diagnoses as the country as a whole. There were some differences between regions in sickness absence with specific psychiatric diagnoses, but they were small and most comparisons were non-significant. Conclusion Sickness absence with psychiatric diagnoses increased between 1994 and 2000 in Norway. The increase was highest in the middle-aged, and in women. Few regional differences were found. That the increase pervaded all stratification groups supports general explanations of the increase, such as changes in attitudes to psychiatric disorders in both patients and doctors, and increased mental distress probably associated with societal changes at a more structural level.