The purpose of this study was to examine the influence of sample concentration and chamber depth on the performance of a real-time analysis computer-assisted semen analysis system, the Hobson Sperm Tracker. Fresh semen samples were provided by patients or donors who attended the author's clinic. The samples were used to estimate total concentration, percentage motility, and sperm kinematics. A considerable variation was found in total concentration and motility recordings between manual and computerized analysis, which was more profound in high-density samples (>80 x 10(6) sperm/mL). The sperm motion parameters were significantly different between low- and high-density samples. This difference could not have been due to sample variation since it was also observed after 1:10 dilution of dense samples. The results indicate that a real-time analysis system can be used clinically for semen analysis over a wide range of sperm concentration. However, high sperm concentration can distort sperm count, motility, and sperm kinematics recordings.