Earlier, we established a relationship between sex hormone receptor concentration in tumor and 5-year survival, on the one hand, and seasonality, on the other. The parameters showed a distinct 6-month cycle. That pointed to certain environmental factors which could synchronize hormone-dependent tumor process in the breast of women living in the North. The present study is concerned with a relationship of 6-month rhythm of tumor growth and latitude of residence. Said rhythm was reliably identified as a parameter of 5-year survival in the Far North (68 deg. northern latitude, p