Objective: The study was performed to evaluate the correlation between sperm motility index, a novel parameter of semen quality, and routine semen analysis parameters by microscopic evaluation. Design: Sperm motility index was measured by an electro-optical device, the Sperm Quality Analyzer (United Medical Systems Inc., Santa Ana, CA). Human semen samples covering the whole span of qualities were analyzed prospectively and simultaneously by both methods. Setting: Samples were collected from patients referred to university hospital infertility clinics. Patients, Participants: Nine hundred sixty-eight semen samples of 812 patients and healthy men were analyzed. Main Outcome Measure(s): Sperm motility index is a measurement of optical density fluctuations caused by motile cells; therefore, a positive correlation was anticipated between its values and semen motility parameters. Results: Sperm motility index values demonstrated statistically significant correlation with motile cell concentration, total cell concentration, and percent motile cells. They were also shown to reliably represent semen quality assessment obtained by two experienced andrologists. Conclusions: The sperm motility index provides a reliable and objective reflection of semen motility parameters and quality.