Evidence for seasonal variation in polymyositis.

Fifty-one cases of polymyositis/dermatomyositis were reviewed in order to investigate seasonal distribution in the onset of the disease. The time (month, season) of first admission of the 51 patients was evaluated as well as the time of initial symptoms onset in 26 cases with duration of the disease which was less than 1 year. In both groups a concentration of cases was found for the months of March, April and May. This concentration was statistically significant (p less than 0.003 and p less than 0.007, respectively). These findings suggest that environmental factors could be involved in the etiology of the disease.