In routine semen analysis of 242 patients the values of sperm concentration, sperm motility and progressive sperm motility were measured with the Hamilton-Thorn semen analyser (HT) and compared to the data obtained by conventional semen analysis according to the guidelines of the WHO. Overall, the HT gave higher values for sperm concentration (mean difference 21.7 +/- 46.2 x 10(6) ml-1, mean +/- SD). Motility values showed a correlation of 0.67 (slope 0.94, P less than 0.001) but were estimated lower by the HT than by conventional analysis (mean difference 7.3 +/- 21.7%); this was caused by the overestimation of sperm concentration. In the range from 0 to 50% motility the HT yielded lower values and higher values from 50 to 100% motility. Progressive motility values of the HT agreed better with conventional analysis (WHO categories a + b): the mean difference of all values was 0.5 +/- 19.5% (r = 0.74, slope = 1.0). The mean lateral head displacement measured by the HT increased with increasing sperm path velocity, but other significant correlations between sperm movement parameters could not be demonstrated. In many instances round cells and debris could not be distinguished from normal sperm. In conclusion, the HT system cannot replace conventional semen analysis in routine diagnosis.