A prospective, randomized double-blind study of 38 children with cystic fibrosis (CF) was designed to evaluate the effectiveness of cimetidine in improving fat absorption and clinical condition. The treatment consisted of cimetidine or placebo, 600 mg/m2 body surface/day, over a 4-mo period. Clinical state, weight, height, skinfold thickness, lung function tests, para-aminobenzoic acid (PABA) peptide test, and plasma lipid and lipoprotein determinations were performed before and after the treatment period. Compared with age-matched healthy children, patients showed decreased cholesterol (150.2 +/- 31.2 mg/dl, mean +/- SD), decreased high density lipoprotein cholesterol (44.1 +/- 11.8 mg/dl), and decreased low density lipoprotein cholesterol (84.1 +/- 25.5 mg/dl) whereas the triglycerides and the very low density lipoprotein triglycerides were slightly elevated (118.2 +/- 33.0 mg/dl and 60.5 +/- 17.5 mg/dl, respectively). Apoprotein B and AI were slightly reduced and Apoprotein AII was in the normal range. After the 4-mo treatment no significant change in clinical condition, weight, or lipoprotein patterns could be detected between the two groups. The total PABA recovery in urine also did not change significantly (36.6 +/- 19.4% of the dosage given before versus 28.7 +/- 12.9% after 4 mo in the cimetidine group). Cimetidine gave rise to bronchoconstriction as shown by an increase in airway resistance (mean increase 14.8%) whereas the placebo group had a decreased Raw with a mean of 8.3%. Patients with CF have a dyslipoproteinemia that was not influenced by cimetidine. We conclude that cimetidine does not improve fat absorption and has, therefore, no place and no benefit in the treatment of children with CF.