Since the withdrawal of phenformin in 1978, the use of metformin has increased from 13,500 to 22,000 patient years/year. During the period 1977-91 a total of 18 cases of metformin-associated acidosis was reported, of which 16 had lactic acidosis. The incidence of reported acidosis and lactic acidosis decreased from 1.50 cases per 10,000 patient years in 1977-81 to 0.24 cases per 10,000 patient years 1987-91, probably due to lower doses doses and reduced usage in the very old. All the reports described patients with several other concomitant diseases, mainly cardiovascular and renal, when the acidosis was diagnosed. It is important continuously to re-evaluate metformin therapy and to stop treatment at the onset of impaired renal or cardiovascular function.