Forty-five patients (25 male and 20 female) over 12 years of age with cystic fibrosis have been studied clinically, radiologically and physiologically. Their mean age at the first visit was 17 years; they were followed for a mean period of 4 years and attended at least every six months. The first symptom which developed before the age of five in 42 of the 45 patients was respiratory. Thirty-two of the 45 patients had severe lung disease (Group III) at the start of the study of the seven patients died during the study. Cough and sputum were almost universal, 23 had haemoptyses and eight pneumothoraces. Staphylococcus pyogenes, Haemophilus influenzae and Pseudomonas aeruginosa were the common pathogens isolated from sputum and the increasing prevalence of the latter was again confirmed. Acquisition of the mucoid strain of pseudomonas signified poor prognosis. Established infection was never eradicated. Forty-three patients had evidence of pancreatic insufficiency; in all but one patient the symptoms were mild and five patients abandoned dietary restriction and pancreatin without ill effect. Seven patients had symptoms of partial bowel obstruction (meconium ileus equivalent) but only one required surgical relief. The liver was enlarged in seven patients and the spleen was felt in three. Three patients had diabetes mellitus. The influence of cystic fibrosis on growth and development is reported--the growth spurt is late in the majority but growth failure is not confined to those with severe lung infection or malabsorption and in these circumstances remains unexplained. Mean weight was low in relation to height and puberty was delayed in both sexes.