The authors describe three cases of fatal acidosis observed in women aged 69, 74, and 68 yr. The striking feature was the high concentration of lactic acid in the serum (a maximum of 44 mM/liter in Case 2), as compared with concentrations (maximum 13 mM/liter) found in 23 cases of shock and uremia. The three patients were diabetics who had been treated with phenformin hydrochloride; all had histories of renal disease and infection. The authors recommend caution in the use of phenformin hydrochloride in the treatment of diabetes when renal function has been impaired by acute or chronic disease.