We hypothesized that patients with cystic fibrosis would have abnormal lithium handling because the genetic defect in cystic fibrosis encodes an ion channel which causes a generalized electrolyte transport disorder in epithelial membranes. Eight patients with cystic fibrosis and eight age-sex matched healthy subjects ingested 600 mg lithium carbonate and had urine and serum lithium levels assessed 24 h later. Compared with healthy subjects, the patients with cystic fibrosis had higher serum lithium levels (0.071 +/- 0.038 vs 0.113 +/- 0.055 mmol l-1, P = 0.03) and had lower fractional renal excretion of lithium (27.5 +/- 14.8 vs 18.8 +/- 9.3%, P = 0.03). Caution should be used in prescribing standard doses of lithium to patients with cystic fibrosis until more definitive data are available.