Thymidine labelled with a position emitter and imaging by positron emission tomography has been suggested to measure tumour proliferation in vivo non-invasively. The present study presents a compartmental model which describes the behavior of [methyl-11C]thymidine in tumour tissue and allows quantification of the data using a proliferation parameter (PP). In nine patients with squamous cell carcinoma of the head and neck, 11C-activity over the tumour and the adjacent blood vessels was measured. The median PP without correction for blood pool activity was found to be 8.61 10(-4) min(-1). With correction this parameter changed from -14% to +40%. We found no correlation between either sex or tumour differentiation and PP. The described model can be used to quantitate [methyl-11C]thymidine uptake in human tumours. Indivdual correction for tumour blood volume is mandatory.