A case of lactic acidosis associated with phenformin therapy for diabetes mellitus is reported, and 34 previously reported cases of lactic acidosis associated with phenformin therapy are reviewed to determine if any predisposing factors to lactic acidosis were apparent. Observations of sex, age, duration of diabetes, pathologic conditions, dosage, duration of phenformin therapy and the onset of symptoms preceding lactic acidosis were made. Renal impairment, urinary tract infections, hepatic impairment, ethanol ingestion and poorly controlled congestive heart failure were found to be predisposing factors to lactic acidosis. The appearance of a syndrome of impending lactic acidosis consisted of anorexia, nausea, vomiting with abdominal pain or lethargy.