Impairment of mucociliary clearance as a result of genetic defects of cilia in the respiratory tract has been recognized as a cause of chronic or recurrent respiratory diseases. Respiratory cilia have been examined by high resolution electron microscopy of nasal and bronchial biopsies from children and young adults from 6 months to 24 yr of age. In this series, 17 children with immotile cilia syndrome have been shown to have deficiencies of dynein arms in the cilia. Ultrastructural analysis reveals a variability of dynein defects from the lack of inner arms, the lack of outer arms, to the complete lack of both inner and outer dynein arms. The spectrum of defects that contribute to dynein-deficient cilia presumably reflects separate genetic determinants, affording further evidence that the immotile cilia syndrome is genetically heterogeneous. Despite ultrastructural differences in cilia, no significant differences are evident in the clinical course of the respiratory disease in affected subjects or in the incidence of situs inversus that affects 50% of subjects.