The aim of this study was to assess the prognostic significance of mucoid and non-mucoid isolates of Pseudomonas aeruginosa (muPs and non-muPs) from the sputa of patients with cystic fibrosis (CF). Eighty-one children with CF who coughed up sputum daily were recruited and followed over 12 months with frequent sputum cultures. At the end of this observation period they were classified to one of three age-matched groups. In 50 mPs was isolated on one or more occasions; 19 grew non-muPs but not muPs, and 12 grew no isolates of Ps aeruginosa. These 81 children and adolescents were followed for a further 8 years or until they died. Twenty-one (42%) of the muPs patients died compared with two (11%) of the non-muPs and one (8%) of the no Ps patients (P less than 0.01). Stepwise regression indicated that forced expiratory volume in 1 second (FEV1) had the main predictive effect but that age, Shwachman score and muPs also had a predictive effect. Identification of mucoid forms of Ps aeruginosa is an unfavorable prognostic factor but the isolation of non-mucoid strains does not appear to be any more important than the isolation of other common respiratory pathogens.