OBJECTIVE To determine the accuracy and precision of a computer-automated semen analyzer (CASA; Hamilton-Thorne Research, Beverly, MA) over a wide range of sperm concentrations. METHODS Prospective study using comparative measurements of sperm concentration and motility were performed using manual and CASA methods of analyses. METHODS A hospital-based Andrology laboratory. METHODS Male partners of couples undergoing infertility evaluation. METHODS Measurements of sperm concentration in the range of 1 to 200 x 10(6)/mL were performed using hemacytometers and CASA-analyzed 20 MicroCell counting chambers (Conception Technologies, Inc., La Jolla, CA). Accuracy was assessed by comparison to the hemacytometer ("gold standard"). The MicroCell counting chambers also were used for manual and CASA sperm motility determinations. RESULTS The CASA-analyzed 20 Microcell Chamber most precisely determined sperm concentration in the 1 to 200 x 10(6)/mL range (intraclass correlation coefficient = 0.94). The hemacytometer was least precise (intraclass correlation coefficient = 0.91). There was a high degree of correlation between the manually and CASA-analyzed MicroCell Chambers in the sperm concentration range of 20 to 149 x 10(6)/mL. Above and below this range, correlation was lower. Motility was determined most precisely using the CASA-analyzed 20 MicroCell Chamber. CONCLUSIONS The Hamilton-Thorne CASA used with the 20 MicroCell Chamber is a precise and accurate method for determining sperm concentration and motility of semen specimens with concentrations in the 20 to 149 x 10(6)/mL range. This same combination of CASA and MicroCell Chamber provides precise sperm motility results.