We evaluated the relationship existing between the site of intestinal lesions and systemic acid-base balance in 78 patients with active Crohn's disease. Patients with enteritis had a normal acid-base balance, while mild and moderate metabolic alkalosis were present in enterocolitis and colitis. These findings appeared to be related to the electrolyte fecal losses. In enteritis fecal sodium and chloride concentrations were within the normal range (Na 29.2 +/- 18.5; Cl 16.6 +/- 11.2 mEq/l) while in colitis they were significantly higher (Na 52.8 +/- 20.8; Cl 29.6 +/- 12.7 mEq/l). Intermediate values were observed in enterocolitis. The fecal potassium concentrations were similar in the three groups of patients, with a slightly lower concentration in enterocolitis and colitis. The fecal K/Na ratio was normal in enteritis and reversed in enterocolitis and colitis. This study suggests that a relationship exists between the site of lesions, fecal electrolyte losses, and systemic acid-base balance in Crohn's disease. Systemic metabolic alkalosis and an abnormal fecal K/Na ratio occurred in patients with colonic involvement, indicating the important role played by the colon in acid-base and electrolyte homeostasis.