The seasonal pattern of prostatic acid phosphatase (PAP) and prostate specific antigen (PSA) in nonmalignant males was investigated. Serum levels were measured in 1,540 men during a 3-year period with radioimmunoassay methods using monoclonal antibody techniques. All of the tested individuals were free of prostatic malignancy. During each of the 3 years, PAP ans PSA showed a rise, especially in spring. The mean PSA level in spring showed a statistically important difference when compared with winter, fall and summer mean levels (p less than 0.05). However, no significant difference of PAP levels was estimated seasonally in the 3 years, which shows that an important marker of prostatic cancer can vary with seasons.