The effects of an artificial diet on growth were examined in a controlled trial on 43 patients with cystic fibrosis. 28 patients received the diet for 12 months. Comparison was made between their growth before, during, and after dietary treatment. Growth changes were also assessed in 15 other patients who received a conventional diet. Artificial dietary treatment led to some improvement in height, weight, subscapular skinfold thickness, and a disproportionate advance of bone age for the group as a whole. Only 10 (36%) patients had a 'clinically' significant improvement in height, weight, or both--i.e. more than 0.5 standard deviation score. Greatest improvements were in young and mildly affected patients. It is proposed that the future use of such an unpleasant and expensive diet should be restricted to a few selected cases, rather than given as routine treatment.