The proportion of spermatozoa exhibiting the vigorous motility behaviour termed 'hyperactivation' (HA) has been shown to be increased following removal of seminal plasma and stimulation with chemical agents such as pentoxifylline. The aim of this study was to examine the relationship between the proportion of HA in cryopreserved semen samples from sperm donors and the corresponding pregnancy rates achieved by donor insemination. Cryopreserved samples from 20 men were incubated in the presence or absence of 3 mM pentoxifylline for 1 h and the %HA determined in each sample. The relationship between pregnancy rate, the proportion of HA spermatozoa in control and pentoxifylline-treated groups and the change in %HA following pentoxifylline treatment (delta HA) as well as the mean semen characteristics for each donor [sperm count, motility (%), motility index, normal morphology (%), post-thaw motility (%) and post-thaw motility index] were examined by logistic regression of the occurrence of clinical pregnancy with each insemination. Both delta HA and mean post-thaw motility index were significantly related to pregnancy rates and together accounted for 64% of the observed variation in pregnancy rates.