Pre-emptive pharmacogenetic (PGx) testing combined with clinical decision support is a promising new strategy for making pharmacotherapy safer and more effective. To estimate the number of patients whose therapies could be guided by this approach, we analysed claims data for patients in Austria in the years 2006 and 2007. We calculated the number of patients receiving one or several drugs for with pharmacogenomic guidelines are available (PGx drugs). The cohort consisted of 6,761,034 patients and was split into four age groups. Patients in the age group >= 65 were prescribed the most PGx drugs, with 72% of the patients receiving at least one PGx drug. 39.1% of all people over 65 received at least one drug metabolized by the three most frequent cytochrome P450 enzymes. Our data indicate that a sizable fraction of elderly patients could profit from the implementation of pre-emptive PGx testing and decision support.