In drug utilisation studies, the units of defined daily doses (DDD) and DDD/1000 inhabitants per day standardise for differences in dosage and population size, but not for age-related differences in drug utilisation. There is no consensus as to how age standardisation of DDD data should be carried out. Using cardiovascular drug utilisation data from Sweden and Spain, the current study compared the outcome of different methods of age standardisation. Both indirect methods (based on a comparison of observed and expected drug usage) and direct methods (using different weighting for the age categories) were used. The largest impact of standardisation was seen for diuretics. The crude rate for men and women combined was 26 DDD/1000 inhabitants per day in Costa de Ponent and 98 DDD/1000 inhabitants per day in Värmland. The corresponding figures when standardising the Costa de Ponent population were 26 and 58, respectively. Using the equivalent average rate (EAR) method, the rate for Värmland was 129 DDD/1000 inhabitants per day. Lesser but still important differences were found for beta-adrenoceptor and antihypertensives. Thus, the results of standardisation differ depending on which method is used and which drugs are evaluated. EAR is recommended for direct standardisation because of its ease of use and because it does not require the choice of a standard population.