Flagellar dyskinesia is characterized by abnormal sperm movement parameters and a negative sperm mucus penetration test. It is associated with structural pathologies of the axonemal complex (lack of outer dynein arms), of the periaxonemal complex (sliding spermatozoa and periaxonemal dyskinesia), or of both structures (short flagella). Even during in vitro fertilization, dyskinesia prevents the spermatozoon from getting through the egg vestment. However, in some cases, fertilization has been achieved using subzonal insemination. Flagellar dyskinesia is therefore an interesting model for investigating the role of sperm movement in the fusion process between the spermatozoon and the oolemma. Thirty-one patients requiring assisted fertilization were included in the study. Fifteen had spermatozoa in which the flagellum lacked outer dynein arms, 11 had anomalies of the periaxonemal complex (five with sliding spermatozoa and six with periaxonemal dyskinesia) and five had spermatozoa with short flagella. Seven men who produced spermatozoa with normal movement were selected as controls. Movement was evaluated using a computer-assisted analyser, and penetration was assessed using zona-free hamster eggs. At 37 degrees C, in semen, the dyskinetic spermatozoa had reduced straight line and curvilinear velocity and lateral head displacement compared with controls (P < 0.01). In the Percoll-selected sperm suspension, the only difference was that spermatozoa with periaxonemal anomalies maintained a narrow lateral head displacement compared with the controls (P < 0.001). After 3 h of incubation at 37 degrees C, the lateral head displacement of dyskinetic spermatozoa had not changed, while that of the controls showed a significant increase (4.5 to 5.6 microns; P < 0.05). The results from the sperm penetration assay for the spermatozoa lacking outer dynein arms were lower than those of the controls (47% versus 77%; P < 0.05) and the results for sliding spermatozoa and spermatozoa with periaxonemal dyskinesia were even lower (25% and 34%, respectively; P < 0.01). The fertilization rates after subzonal insemination were 46.5% for spermatozoa lacking outer dynein arms, 36.1% for spermatozoa with short flagella, 24.8% for sliding spermatozoa and 17.3% for spermatozoa with periaxonemal dyskinesia. There was a significant correlation between the curvilinear velocity of the Percoll-selected sperm suspensions and their fertilization rates after subzonal insemination (r = 0.5; P < 0.05) and their sperm penetration assays (r = 0.7; P < 0.001). The data provide evidence that sperm velocity is correlated with the ability to fuse with the oolemma.