The relationship and degree of association between the percentage of sperm swelling (HOS-test) and conventional semen variables was investigated in 263 consecutive ejaculates. The semen samples were exclusively obtained from men suspected of primary infertility. It was found that the correlation coefficients (Spearman's rho) followed the order: percentage of progressive motility at 3 h greater than count/ml greater than percentage of total motility at 3 h greater than percentage of normal spermatozoa. Of the three morphology sub-classes considered (sperm head, mid-piece and tail abnormalities), only mid-piece abnormalities correlated with the outcome of the HOS-test (rho = -0.409). Linear relationships between HOS-test results and sperm motility and morphology, but not sperm count, were indicated by LOWESS-smoothing. However, a linear relationship between the HOS-test, sperm count and a 'functional index' combining the conventional semen variables could be demonstrated after normalization of the data. Our findings suggest that the HOS-test may be of value in assessing the functional integrity and viability of spermatozoa; however, its prognostic power for fertility is probably not different from that of conventional semen variables.