Sperm motility is clearly essential for fertilization both in vivo and in vitro. Motility is necessary for successful sperm transport, a step that is bypassed with in vitro fertilization. Recently, increasing attention has been paid to the objective evaluation and characterization of sperm motility more than simply determining the total proportion of motile spermatozoa. The purpose of computerassisted semen analysis (CASA) is to provide values for sperm concentration and sperm motility more rapidly and accurately than those obtained with traditional semen analyses methods. The objective of our experiment was to investigate the effect of specific aspects of sperm movement, such as the velocity of progression and the actual pattern of movement, to the fertilizing capability of sperm. Frozen semen samples of 10 HF breeding bulls were used in the study. For the motility analyses, Medealab CASA system (Medealab, Germany, Ver. 4.1) was used, and the velocity parameter of VCL (curvalinear velocity, μm s^-1), VSL (straight line velocity, μm s^-1), and VAP (average path velocity, μm s^-1) were evaluated and compared with the Day 30 and 75 non-return rates (NR30 and NR75). For every sample, a total of 10 fields were examined for 8 s using a disposable 20 micron capillary chamber (CellVision, USA) giving a total of 1165 to 2831 cells evaluated. Chi square analysis, analyses of variance and linear correlation coefficient was applied to the statistical evaluation and comparison of the results. Data are based on weighted values. From the same batch of the analyzed frozen semen, a total of 8099 females were inseminated in more than 100 farms with a total of 6590 animals being positive for pregnancy at Day 30 and 4525 animals at Day 75. Within the bulls, differences were found in the values of NR30 and NR75 (P < 0.05). Our data indicate very strong differences between the males’ NR30 and NR75 values (NR30: 65.6% ± 13.04 to 79.6% ± 11.17; P < 0.001 and NR75: 37.8% ± 10.38 to 58.3% ± 15.53; P < 0.001) reflecting the individual differences in the fertilizing capability of the males. All velocity parameters show very high correlation with strong significance both non-return rates but the best values belong to VAP (NR30 and NR75; P < 0.02). Our data indicate that the bulls with lower VCL (25.51 ± 33.04 to 79.54 ± 58.03), VSL (11.35 ± 19.45 to 36.36 ± 35.71), and VAP (12.67 ± 19.06 to 41.75 ± 34.45) values showed lower fertilization rates both at NR30 and NR75. Computer and video technologies have advanced rapidly in recent years; thus the capability and accuracy of the latest versions of CASA systems are considerably better and they give more information about the different motion characteristics of spermatozoa. Because of the vital role of sperm motility in the reproductive process, such systems will enable us to move into a new era of diagnostic andrology and predict the fertilizing capability of semen. Supported by NKFP-Grants 4/040/2001 and 4/031/2001.