The efficacy and safety of mono-octanoin, a cholesterol solvent for the direct dissolution of stones in the biliary tract, was assessed by collating case reports on 343 patients provided by 222 physicians who used the material between 1977 and 1983. Most patients had previously undergone cholecystectomy, with common duct exploration, the majority within the preceding six weeks. In most, sphincterotomy was impossible or if carried out, had not induced stone passage. Stone dissolution was considered preferable to surgery, especially in patients who were frail, elderly, or had multiple medical problems. Treatment was unequivocally successful in 88 patients (26%) and was a valuable adjunct to interventional treatment in another 29 subjects (8%). In 70 patients (20%), calculi became smaller, but remained within the biliary tree. Thus, mono-octanoin was judged to have been useful in 54% of patients. Treatment was ineffective in 124 cases (36%). In the remaining 32 patients (9%), treatment was aborted because of side effects. Such side effects were common, occurring in 67% of cases, and in 41% of patients they were multiple. Abdominal pain was the most common complication. Other side effects reported were nausea, vomiting, and diarrhoea. Side effects were usually dose related and responded to reduction in infusion rate. Side effects were life threatening in 12 patients (5%), but there were no permanent sequelae and no deaths occurred. These data indicate that mono-octanoin is moderately effective, generally second line, but sometimes first line, treatment for retained biliary duct calculi.