The CYP3A4 enzyme contributes to the disposition of more than 60 therapeutically important drugs and displays marked person-to-person variability of the catalytic function. However, the extent of genetic contribution to variability in CYP3A4 activity remains elusive. Recently, we showed that a comparison of between- (SDb2) and within-person (SDw2) variances provides an estimate of the genetic component of variability in drug disposition. The aim of the present analysis was to assess the genetic control of CYP3A4 activityin vivo. A computerized literature search was conducted covering 1966 to September 1999 to identify studies reporting repeated administration of CYP3A4 substrates. The genetic contribution (rGC) to disposition of each CYP3A4 substrate was obtained by the formula (SDb2−SDw2)/SDb2. TherGCvalues approaching 1.0, point to overwhelming genetic control, whereas those close to zero suggest that environmental factors dominate. A total of 16 studies with 10 different CYP3A4 substrates were identified (n= 161 subjects). TherGCfor hepatic CYP3A4 activity as measured by midazolam plasma clearance or the erythromycin breath test was 0.96 (0.92–0.98) (95% CI) and 0.89 (0.65–0.98), respectively (P<0.05). The point estimates ofrGCfor composite (hepatic + intestinal) CYP3A4 activity measured after oral administration of cyclosporine, ethinylestradiol, ethylmorphine, nifedipine and nitrendipine, ranged from 0.66–0.98 (median: 0.83) (P<0.05). Cyclosporine data suggested a higher genetic control of CYP3A4 at night than during the day. These data indicate that further molecular genetic investigations are warranted to identify genetic variants atCYP3A4or elsewhere in the genome which contribute to regulation of CYP3A4 activity.