Finger clubbing was measured in 73 of 105 patients with cystic fibrosis undergoing full assessment. The sign correlated well with the chest x ray score and indices of pulmonary function and infection but not with weight, height, age, liver function, or degree of fat malabsorption. The presence of clubbing suggests appreciable pulmonary involvement. Most probably its progression indicates a deterioration in pulmonary state. In both instances increased efforts should be made to treat the infection.