The onset of activity of antidepressants is an aspect of their efficacy which is of practical as well as theoretical importance. Using medline a review of the literature published since 1975 was carried out in order to find out if a specific pattern can be observed. The authors point out what problems are most often encountered and illustrate the most common pitfalls. In the absence of a straightforward onset estimation, indirect arguments prevail which have several shortcomings. Methodological problems make it impossible to draw a clear conclusion.