In a pilot study including 18 patients between 20 and 39 years old serum prostate specific antigen (PSA) was evaluated before as well as 1 and 7 days after ejaculation. In 13 patients who had more than 0 ng./ml. PSA before ejaculation (mean 1.44 ng./ml.) there were statistically significant decreases in serum PSA down to 0.17 ng./ml. and 0.29 ng./ml., respectively (p = 0.0001). These results suggest a physiological relationship between ejaculation and decreased serum PSA levels. Due to the clinical significance of PSA in the diagnosis and monitoring of prostate cancer, further studies are needed including men of risk age and untreated prostate cancer patients.