We have retrospectively analysed the sperm characteristics of 416 consecutive healthy young men who presented themselves in the past 19 years as candidate sperm donors. Ejaculate volume increased slightly ( P = 0.067), and average sperm concentration decreased ( P = 0.035) by 12.4×lO6ml over the observation period, so that sperm count per ejaculate remained unchanged ( P = 0.91). In contrast, sperm morphology ( r = −0.23, P < 0.0001), rapid progressive motility ( r = −0.42, P < 0.0001) and total motility ( r = −0.33, P < 0.0001) presented an important and time-related decrease. When a quadratic model was used rather than a linear one to analyse the data on rapid progressive motility, there appeared to have been no further decline since 1990. The average proportion of spermatozoa with normal morphology decreased from 39.2% in the period 1977–1980 to 26.6% in 1990–1995 ( P < 0.0001), and the mean percentage of spermatozoa with rapid progressive motility decreased from 52.7 to 31.7% ( P < 0.0001). The percentage of candidate donors with sperm characteristics below the 5th percentile cut-off value of a normal fertile population increased from 13 to 54% during the observation period ( P < 0.0001). Since the technique of semen analysis has remained essentially unchanged in-so-far as has been practically possible, as has the method of recruitment of candidate sperm donors, the observed deterioration of sperm characteristics is considered to reflect degeneration of sperm production among men aged between 20 and 40 years.