Delivery rates and plasma concentrations vary among patients treated with fentanyl patches. Absorption and urinary excretion characteristics of fentanyl were studied in patients undergoing palliative care. Almost 500 patches were analyzed for residual fentanyl content. Fentanyl and norfentanyl levels were determined in the urine of 50 patients. General and mixed effects linear regression models were established for the relationship between fentanyl dose rate and urinary excretion and to incorporate influencing factors. For different patch nominal dose strengths, wide but comparable variability in estimated dose rate and delivery efficiency was observed (coefficients of variation of 15% to 17%). Fentanyl delivery efficiency was 8.5% higher for patches of 25 microg/h as compared to 75 microg/h and, accordingly, 7.5% for patch application on the arm as compared to the leg. Urinary fentanyl and norfentanyl concentrations varied considerably. The general linear model revealed a positive effect of the calculated transdermal dose rate on urinary fentanyl levels, explaining 34% of the variability (P < .0001). In addition, gender (P = .04) and type of cancer pathology (P = .03) exerted significant effects on the linear model, explaining 40% and 64% of the variability, respectively. Delivery efficiency of fentanyl patches can vary substantially, possibly leading to either underdosing or overdosing.