The several genetically-determined structural defects of cilia that cause ineffective mucociliary clearance in Kartagener's syndrome and related diseases (for which the group name Primary Ciliary Dyskinesia is thought most appropriate) are compared with structural defects of flagella of mutant Chlamydomonas, about which biochemical information is also available. This comparison suggests that the effective activity of several or many genes may be essential for formation of each of the major components of the ciliary axoneme, and that Primary Ciliary Dyskinesia may result if a patient is homozygous for ineffective genes at any one of these gene loci. However, the mutants often appear to be "leaky", with incomplete effects; in addition, structural abnormalities of human cilia are relatively common in apparently normal individuals. The relationship of axonemal defects to disorientation of ciliary bases is questioned and the possibility is raised that such disorientation may result from morphogenetic disturbances through infection rather than from genetic origins.