Inconsistent seasonal patterns in peak presentation of acute lymphoblastic leukemia have been reported but no formal synthesis of published reports has been attempted to date. We extracted monthly tabulations of cases from publications and reanalysed using an angular methodology and the von Mises distribution. Twenty-four articles from 11 countries in locations ranging from 35.05 degrees S to 65.01 degrees N were identified. Formal synthesis established weak evidence for seasonality in adults and children. Were seasonality present, and arising from a climatic determinant, one might anticipate peaks of an increasing magnitude as the associated latitude moved away from the equator. No such pattern was apparent.