Forty-five patients with clinically definite multiple sclerosis (DS) with onset in the period studied, were reviewed retrospectively. The time of the first exacerbation after the onset, but not the time of onset exhibited seasonal variation (p = 0.003), as 76% of the exacerbations occurred in the winter months. On review of all 148 cases of clinically definite DS in the department's records, a seasonal variation was found of both the time of onset (p = 0.047) and the time of the next exacerbation (p = 0.0004). In previous studies different seasons of peak disease activity were found. These differences may be caused by different methods or by differences in the local factors, which influence the course of the disease. The seasonal variation of the frequency of the disease manifestations is probably caused by a variation in environmental factors. In this study, the importance of infections could not be evaluated.