Adverse drug reactions (ADRs) to phenformin and metformin reported to the Swedish Adverse Drug Reaction Committee during 1965--77 were analysed in relation to sales and prescription data. The biguanides accounted for 0.6% of all reported adverse drug reactions but for 6% of the fatal cases (all phenformin). Sixty-four ADRs to phenformin and eight to metformin were classified as causal relation "probable" or "not excluded." Fifty-one of these reactions (71%) were lactic acidosis, all but one being reactions to phenformin. After 1973 phenformin was prescribed less in Sweden and metformin became predominant. A nationwide prescription survey during 1975--6 disclosed no differences in age and sex between patients receiving phenformin and metformin. The mean daily doses prescribed in 1976 were 74 mg of phenformin and 1.5 g of metformin. The numbers of ADRs to the two drugs reported during 1975--7 were related to use. The relative incidences of ADRs reported for phenformin and metformin did not differ. Significantly more cases of lactic acidosis and deaths were reported for phenformin.