Nasal epithelial cells, dissociated from nasal polyps of three cystic fibrosis patients, were cultured on collagen gels until all cilia were lost, and then the cells were released in suspension as cell sheaths to form aggregates and vesicles. Cilia reappeared after 1 week. The number of ciliated cells increased progressively and within 1 month large areas of the aggregates were ciliated. These newly formed cilia were ultrastructurally normal. The ciliary activity as well coordinated and resulted in continuous movements of the aggregates in the culture medium. In the supranuclear cytoplasm the different ciliogenetic precursors were found: fibrogranular aggregates with occasionally small tubules, deuterosomes with procentrioles, kinetosomes, basal bodies, and short cilia. Initial stages of ciliary shaft formation, such as elevation of the apical membrane above the basal bodies without axonemal ultrastructure, could be well documented. The results indicate that in cystic fibrosis, ciliogenesis and the resulting ciliary activity in vitro are indistinguishable from those seen in cells from control tissue.