Changes in serum tumour marker levels in non-seminomatous germ cell testicular tumours (NSGCTT) are used to monitor tumour growth and response to treatment. A novel method of calculating the actual tumour marker production (TMP) per day is reported; estimation of the rate of change of TMP is a measure of the tumour growth or regression rate. TMP is calculated mathematically from the rate of increase in serum marker level and its natural half life. TMP is assumed to be proportional to the number of marker producing cells in the tumour. TMP was calculated over the time between orchidectomy and the start of chemotherapy. The rate of increase in TMP with time is expressed as the marker production doubling time (MPDT) and is a measure of the growth rate. In a group of 51 patients with metastatic NSGCTT, TMP varied from 0.012 to 5985 iu/l/day (AFP) and 0.08-5404 iu/l/day (HCG). MPDT varied from 0.5 to greater than 80 days (45 cases) for AFP + ve patients and from 1.8 to greater than 80 days (34 cases) for HCG + ve patients; greater than 80% of cases had a MPDT less than or equal to 32 days. In 45/51 (88%) patients, there was no discrepancy in MPDT between markers. The use of changes in serum marker level to follow tumour progression and regression is simple, but the calculation of actual TMP provides clearer information about the change in number of marker producing cells and can be used as non-invasive method for measuring the tumour growth rate of metastatic disease and response to treatment.