To study the influence of drinking water composition on the risk of myocardial infarction, the following study was conducted: The cases (C), men 30-64 years of age, had been discharged with a first acute myocardial infarction (AMI) from Kotka Central Hospital. The hospital controls (HC), matched for age and type of community, were selected for each case among surgical patients. Population controls (PC), matched for age and municipality, were drawn for each case from the population register. Subjects submitted a sample of their drinking water and a filled-in questionnaire. After exclusions, a series of 50 C-HC and 50 C-PC pairs was finally constructed. The point estimate of relative risk (RR) for the association between low F (less than or equal to 0.1 ppm) and increased risk of AMI was 3.0 in the C-HC series. In the C-PC comparison, RR was 4.4 RR for low Mg (less than or equal to 1.2 ppm) was 2.0 in the C-HC comparison and 4.7 in the C-PC comparison. The results are consistent with the hypothesis that both a low F and a low Mg intake are conducive to atherosclerosis leading to AMI.