In the evaluation of a hypertension treatment program, the end-point surveillance included incidence of acute myocardial infarction and acute stroke identified from hospital in-patient registers and the national mortality register. To ascertain the validity, in-patient records containing the ICD-codes 410-411 and 430-438 were validated. First event of acute myocardial infarction and acute stroke suggested in the in-patient register could be confirmed in 96% and 94%, respectively. In-patient diagnoses of suspected acute myocardial infarction or other acute or subacute ischemic heart diseases, transient ischemic attack and unspecified heart diseases, transient ischemic attack and unspecified cerebrovascular disease revealed high proportions of what in fact turned out to be definite events (11%, 24% and 53% respectively). It is concluded that disease ascertainment for this cohort study claims validation of register data with hospital records.