Aspartame is an artificial sweetener completely metabolized in the gut and absorbed as aspartate, phenylalanine, and methanol. Phenylalanine is thought to mediate or exacerbate hepatic encephalopathy, and an impaired liver may not be able to cope with the ammoniagenic properties of the amino acid constituents, or adequately metabolize methanol. Thus, we compared the clinical and biochemical effects of a single ingestion of aspartame (15 mg/kg) to skim milk (phenylalanine content equimolar to aspartame) and placebo in patients with chronic, alcoholic liver disease in a randomized, crossover study. Aspartame produced an elevation of plasma phenylalanine significantly greater than milk and placebo (Cmax 14.55 +/- 7.38, 10.95 +/- 4.95, 8.84 +/- 4.55 mumol/dl, respectively; p < 0.01). However, quantified encephalopathic changes were observed only with milk (p < 0.05). Plasma aspartate, methanol, formate, and ammonia levels remained unchanged after all treatments. The lack of clinical derangements in encephalopathic indices, methanol accumulation, or biochemical changes in liver status suggests that a single large dose of aspartame (representing 5 times the average daily intake of adults) may be used safely by patients with chronic, stable liver disease.