Deoxycytidine kinase (dCK) is essential for the phosphorylation of gemcitabine and can predict response to gemcitabine in vivo. Conventional Competitive Template-Reverse Transcriptase-Polymerase Chain Reaction (CT-RT-PCR) was correlated with real time PCR using a Light Cycler (LC) with SYBR-Green detection to enable rapid and sensitive detection of dCK mRNA expression. We used cDNA from human xenografts to establish a relation between dCK activity and gemcitabine sensitivity. A significant correlation of LC-PCR was found with CT-RT-PCR (Pearson: r = 0.956; P < 0.0001), enzyme activity (Pearson: r = 0.972; P = 0.003) and gemcitabine sensitivity (Pearson: r = 0.695; P = 0.048). The LC-PCR was also applied to needle biopsy specimens. In bladder tumors a similar correlation was found, while esophageal tumors with a high dCK expression responded to gemcitabine treatment. The LC is a rapid and reliable method for quantitation of dCK mRNA levels in tumors to predict clinical gemcitabine sensitivity.