Variation in semen measures was assessed bimonthly in 15 healthy men over a 6-month period. Average within-subject coefficients of variation for sperm count, morphologic features, motility, and semen volume demonstrate considerable variation. Each day of abstinence increased the volume by an increment of 0.62 ml, the count by 17.6 X 10(6)/ml, and motility by 1.2%. Sperm count and semen volume correlated positively within subjects. Intraclass and serial correlations of semen measures provided an empirical clinical guide to the number and spacing of specimens. Three specimens spaced no more than 2 weeks apart established a reliable semen profile. Findings can be applied to basic research and clinical practice.