BACKGROUND Different follow-up times and methods in return to work (RTW) research make it difficult to compare results between studies, and not all intervention effects and determinants may be constant over time. OBJECTIVE This study aimed to describe the RTW process of a population of long-term sickness-absent workers over a 3-year period in terms of the effect of selected determinants over time. METHODS A total of 7780 sickness-absent persons were registered by social workers in six different municipalities and were followed up for 2 to 3 years. Estimates from multiple logistic regression analyses conducted for every 4 weeks were plotted against time to identify changes in the effects of selected determinants. RESULTS After 1.5 years, 55.2% of the population had returned to work and this level was maintained through the remaining follow-up period. All the included potential determinants were found to be significantly related to RTW at 1 and 3 years. The effects of sex, ethnicity, and income were found to be nearly constant over time. The effects of municipality, diagnosis, and age changed markedly over time and mostly during the first year. CONCLUSIONS RTW increased during the first 1.5 years after which a steady level was maintained. The effect of diagnosis, age, and municipality changed markedly over time.