Since Belgian law recently set a limit to morphine concentration detectable in blood and urine while driving a vehicle, questions arose about the implications for the medical use of opiates. We determined morphine concentrations in whole blood and urine by gas chromatography–mass spectrometry in 15 patients on continuous intrathecal morphine administration. Effects on blood and urine concentration after water intake and the correlation with the intrathecal morphine daily dose were also evaluated. Our results confirm that, in all patients examined, the legally determined maximum blood morphine concentration of 20 ng/ml was never exceeded. Even patients on high intrathecal morphine dose schedules did never reach the maximum legal blood concentration. However, morphine concentration in urine reached levels which exceeded by far the legally determined maximum concentration of 300 ng/ml. Although legal actions against driving under the influence of morphine can only be taken after a positive urine and a subsequent positive blood sample, drivers on intrathecal opiates must be aware of the possibility of a positive roadside drug test.