Infertile men who had 3 or more semen analyses performed in one laboratory were placed in 2 groups (I) oligozoospermic group (n = 106), mean sperm concentration between 1 and 20 million/ml (II) asthenozoospermic group (n = 71), mean sperm concentration greater than 20 million/ml, and mean motility less than 60%. With increasing durations of abstinence from ejaculation before the tests there were significant increases in semen volume and sperm concentration. Semen volume increased over the first 4 days to a similar extent in both groups. Sperm concentrations increased over 15 days, but the effect of abstinence was much greater in the asthenozoospermic group than in the oligozoospermic group (14% compared with 1.4% of the within subject variation). Significant changes in results accompanied repeated testing, notably rises in sperm concentration and motility. Sperm motility was lower in winter and higher in summer in both groups and also, but to a lesser extent, in artificial insemination donors who collected semen in the laboratory. CONCLUSIONS duration of abstinence, the elapse of time and seasonal temperature changes affect semen analysis results, and therefore controls for these variables must be incorporated in any therapeutic trial for male infertility. On the other hand, they only account for a small proportion of the total variability and thus routine correction of results would not greatly improve the value of semen analysis in the prediction of fertility. Furthermore because differences in the duration of abstinence have only a small effect on sperm concentration in oligozoospermic men, restricting sexual intercourse to the time of ovulation may not enhance fertility.