Ultrastructurally atypical bronchial cilia are studied and semiquantitatively analysed in 24 children suffering from recurrent respiratory tract infections with or without bronchiectasis. In patients with Kartagener's syndrome normal-looking and shortened dynein arms are present at some axonemal microtubular doublets. This finding suggests that the polymerization or assemblage of dynein molecules on microtubules only is defective but not totally lacking. Bilateral, local and partial absence of dynein arms is demonstrated in some of the patients with acquired unilateral bronchiectases. These patients also reveal anomalies of the "9 + 2" microtubular axonemal pattern. It is suggested that these abnormalities of the tubulin-dynein system are local and acquired defects that may impair bronchial mucociliary clearance. None of the patients with pneumonia and asthma or with cystic fibrosis studied show any anomalies of the dynein arms. However aberrant axonemal microtubular patterns and other ciliopathies such as naked axonemes and megacilia are present at times in these patients. We postulate that these atypical cilia are secondary acquired abnormalities. Only some patients with bacterial or viral pneumonia demonstrate a partial lack of dynein arms in bronchial cilia. Other ciliopathies such as megacilia, naked and intracytoplasmic axonemes and apical blebs are more frequent and more common in these patients. We suppose they manifest a secondary and rather aspecific pathogenic influence upon the bronchial ciliary substructure.