In Sweden, acute myocardial infarction (AMI) incidence has been found to differ considerably between the neighboring counties of Stockholm and Gävleborg, with an increase in Stockholm during the 1970s. We estimated the AMI incidence in Stockholm in 1973 and in both areas in 1981. To determine if there were differences in the validity of hospital discharge diagnoses, random samples of AMI patients were examined for diagnostic criteria for AMI. In both genders, AMI incidence was higher in Gävleborg than in Stockholm (relative risk (RR) 1.34 for men and 1.21 for women) and increased in Stockholm from 1973 to 1981 (RR 1.21 for men and 1.29 for women). The proportion of patients fulfilling the diagnostic criteria for AMI was similar in both areas in 1981 but 10% less in Stockholm in 1981 than in 1973. These results suggest that differences in the validity of hospital discharge diagnoses cannot explain the geographical differences in AMI incidence, but that this may have contributed to the increasing incidence seen in Stockholm county.