A 35-yr-old infertile man with chronic sinobronchial disease and dextrocardia (Kartagener's syndrome) was found to have immotile sperm and motile nasal cilia in vitro. Ciliary beat frequency in vitro was normal, but in vivo nasal mucociliary clearance was markedly prolonged. Quantitative electron microscopy demonstrated a severe reduction in spermatozoal outer and inner dynein arms compared with normal (p less than 0.001) but normal numbers of outer doublets, central microtubules, and radial spokes were seen. In 2 samples of nasal cilia collected 14 months apart, the number of inner dynein arms was significantly reduced from normal (p less than 0.001), but normal numbers of radial spokes and microtubule structures were seen. Ciliary outer dynein arms were slightly reduced in 1 specimen (p less than 0.001) but were normal in the other. It is suggested that the reduction in the number of ciliary inner dynein arms does not affect ciliary motility in vitro but that, under the increased load of mucus in vivo, this defect prevents the cilia from functioning normally. The difference in axonemal ultrastructure between cilia and spermatozoa from the same patient further suggests a separate genetic control of their structural components.