The usefulness of dried blood spot (DBS) sampling for therapeutic drug monitoring of tacrolimus was investigated with renal transplant patients. There was no significant difference between the concentrations (ranging 3.33-53.9 mug/l) of 34 samples of 26 stable renal transplant outpatients, measured both after venous and DBS sampling. DBS sampling is easy to perform because concentrations with and without nurse assistance did not significantly differ. No significant difference was found between tacrolimus concentrations in 20 duplicate DBS samples before and after postal transport. DBS seems promising for routine patient monitoring.