In a previous study of patients with acute cholecystitis, we demonstrated equal efficacy with a broad spectrum penicillin (piperacillin) and a penicillin plus amino-glycoside combination. Whether a single agent broad spectrum penicillin is adequate treatment for more severe infections, such as acute cholangitis, however, is still unclear. We, therefore, conducted a three center, prospective, randomized trial to determine whether or not a broad spectrum penicillin alone is adequate therapy for patients with acute cholangitis. During a 36 month period, 96 patients with sepsis and biliary obstruction were randomly assigned to receive either piperacillin (n = 49) or ampicillin plus tobramycin (n = 47). The two groups receiving antibiotics were similar with respect to all clinical and laboratory parameters. The incidence of blood cultures with positive results (20 versus 21 per cent) and underlying malignant lesions (51 versus 62 per cent) was also similar between the two groups. The percentage of patients with a clinical cure or significant improvement was the same in the two groups (69 versus 70 per cent). However, there was a significant difference in the cure rate between patients with benign and malignant biliary obstructions (83 versus 59 per cent, p less than 0.01). No significant differences were noted between the two antibiotic groups with respect to drug toxicity, but patients with malignant conditions were more prone to antibiotic related toxicities (2 versus 19 per cent, p less than 0.05). These data suggest that outcome of treatment in patients with acute cholangitis is similar with either a broad spectrum penicillin or a penicillin plus aminoglycoside combination and is dependent upon the nature of the biliary obstruction.