In order to study the influence of sexual abstinence on sperm concentration and semen volume, we have evaluated 195 semen samples of healthy individuals, grouped as follows according to abstinence length: A) 2-4 days; B) 5-7 days; C) more than 7 days. Differences between semen samples collected at home or at the laboratory were also studied. Semen volume was smaller in group A than in group B and C (p less than 0.01), while no significant difference was found in sperm concentration among the three groups. The place of collection did not affect semen characteristics. These results suggest that while a shorter abstinence is associated with a smaller semen volume the length of abstinence does not affect sperm concentration.