The number of anaerobes in selected sputum samples from patients with cystic fibrosis (CF) was investigated. When cultured by a semi-quantitative method, 26 (23.85%) of 109 sputum specimens from 21 CF patients contained greater than 10(5) cfu of anaerobic organisms/ml. Nine (42.7%) of the 21 patients produced sputum containing such concentrations of anaerobes on at least one occasion. Anaerobes were isolated from repeated sputum specimens from five patients. The anaerobes most often isolated were Bacteroides disiens, pigmented Bacteroides spp. and anaerobic gram-positive cocci. Anaerobes were isolated more often from sputum liquefied by sonication than from unliquefied sputum, suggesting that they were unlikely to be oropharyngeal contaminants.