This retrospective study reviews the patterns of P. cepacia pulmonary infection in 75 of a total of 872 mainly adults with cystic fibrosis, registered here during the 4 years 1987-1990; 35 (47%) were female. During this period, 55 patients acquired P. cepacia and the annual incidence and prevalence rates have remained between 1.6 and 3.1%, and 4.1 and 5.9%, respectively. The mean age at the time of the first isolation of P. cepacia was 23 years, ranging 11-45 years. Sixty-eight percent of the initial isolates were multi-resistant (sensitive to fewer than three of 15 anti-pseudomonal agents). Prior to acquisition of P. cepacia, 28 (50.9%) patients already had severe lung disease and only three had normal lung function. Infection was transient in 39.1% of patients. Initial multi-resistance of P. cepacia to anti-pseudomonal agents was significantly associated with persistent infection. Clinical outcome was unaffected by age, sex and early intravenous antibiotic therapy but was significantly adversely affected by increasing severity of lung disease at the onset of P. cepacia infection and by initial multi-resistance, and persistence of the organism. Thus, all patients with normal or mild lung disease at the outset of infection have remained clinically stable, whereas, only six of 28 patients with severe disease remained stable, three of whom were transiently infected with P. cepacia. The prevalence of P. cepacia at the time of death fluctuated between 12.5% and 26.9% during the study period.