The study of 17 infertile men has led to define a new entity of sperm pathology as part of the more general field of flagellar dyskinesias. Sperm parameters of the studied patients and a control series have been first estimated by routine analysis (concentration, motility, morphology). To precise their characteristics, kinetic and ultrastructural investigations, as the zona-free hamster oocyte penetration test, have been performed. Sperm parameters of the studied cases, as revealed by routine analysis, were close to the control group. However, a major kinetic anomaly was found which was characterized by an important decrease of the amplitude of lateral head displacement (1.6 microns vs 5.3 microns, p < 0.001), although the progressive velocity was only slightly impaired (20.3 microns vs 24.9 microns, p < 0.05). Electron microscopy revealed anomalies limited to the peri-axonemal structures such as the outer dense fibers and the fibrous sheath. Rates of sperm-oocyte attachment were normal but rates of oocyte penetration were low (27.7% of decondensed sperm heads vs 85.6%, p < 0.001). Attempts to assisted fertilization with the studied patients (51 cycles of insemination, 8 cycles of in vitro fertilization) were unsuccessful. All these data suggest that the infertility can be attributed to the movement disturbances which should impair sperm propulsion throughout the cervical mucus and the zona pellucida.