A nationwide study of all 3183 female patients with breast cancer in Israel diagnosed over a 7-year period (1960 to 1966) was conducted. Monthly series analysis showed a clear seasonal pattern in the symptomatology of the disease most pronounced in patients younger than 55 years, in all ethnic groups, mainly confined to cases with a nonlocalized tumor at diagnosis. Peaks occurred during spring, and troughs appeared during autumn. It is suggested that the pattern is of endogenous, probably hormonal nature, although environmental factors cannot be excluded.