Background Previous pharmacokinetic studies with docetaxel have mostly used 3-weekly (75 mg/m2 and 100 mg/m2) or weekly regimens (35–40 mg/m2). The pharmacokinetics and radiosensitizing efficacy of weekly 20 mg/m2 docetaxel, has however not been well characterized. We examined the pharmacokinetics of weekly docetaxel when administered with concurrent radiotherapy and compared the results with a 3-weekly 100 mg/m2 regimen. Methods Thirty-four patients with non small cell lung cancer (NSCLC) were included in this study, 19 receiving 100 mg/m2 docetaxel 3-weekly as single therapy, and 15 receiving 20 mg/m2 docetaxel weekly with concurrent radiotherapy. A newly developed HPLC method was used for measuring docetaxel levels, capable of quantifying docetaxel in plasma down to the nanomolar level. Results The HPLC method showed detectable concentrations of docetaxel in plasma even after 72 hours. In the present study we have demonstrated that median docetaxel plasma levels of 3 nM can be obtained 72 hours after a dose of 20 mg/m2. Conclusion The pharmacokinetics of docetaxel is characterized by great inter-individual variability and at some time points plasma concentrations for 20 mg/m2 and 100 mg/m2 docetaxel were overlapping. Extrapolation of these results indicates that radio sensitizing docetaxel concentrations may be present for as long as 1 week, thus supporting the use of 20 mg/m2 weekly docetaxel.