As little information exists on the semen variability in infertile men, this study aimed at analysing the within-subject variability of semen from men with infertile marriages included in an intrauterine insemination (IUI) programme. Five ejaculates from each of 436 men (2180 specimens) were analysed. The within-subject coefficients of variation (CV(w)) were high for all parameters (semen volume, sperm concentration, forward motility and combined parameters), ranging from 0.73 for the total motile sperm count to 0.27 for the semen volume. Nevertheless, within-subject fluctuations were smaller than the between-subject variability, as indicated by high Intraclass Correlation Coefficient (ICC) values, which, however, significantly lowered when <or=3 samples were analysed against five samples. Neither seasonality nor the length of abstinence between 3 and 7 days significantly affected within-subject semen variability, whereas total motile sperm count was significantly and negatively affected at 2 days of abstinence. In 253 cycles of double IUI, performed in two consecutive days, semen volume and parameters including volume decreased in the second sample, but semen quality improved in most cases of oligo- and/or asthenozoospermia. Useful information for clinical purposes is provided.