Tumour growth rates have been measured in metastatic non-seminomatous germ cell testicular tumours (NSGCTT) by estimating the rate of rise of tumour marker production (TMP). TMP was calculated for the time between orchidectomy and the start of chemotherapy in a group of 58 patients with metastatic NSGCTT treated with BEP combination chemotherapy (bleomycin, etoposide and cisplatin). Calculation of TMP (iu/l/day) took account of the continuing clearance of marker from the serum. TMP increased with time in 51 patients and this rise generally appeared to be exponential. The rate of this increase was expressed as the marker production doubling time (MPDT) and is a measure of the tumour growth rate. MPDT varied from 0.5 to > 80 days (45 cases) for AFP +ve patients and from 1.8 to > 80 days (34 cases) for HCG +ve patients. Patients who failed BEP first line therapy had shorter MPDTs than those who responded (AFP P = 0.08, HCG P = 0.003). It was found that patients with a MPDT ≤ 4 days were more likely to fail treatment than those who had a MPDT > 4 days (AFP P = 0.009, HCG P = 0.005). MPDTs were independent of initial serum marker concentration. Patients with small volume disease had longer MPDTs than patients with large volume disease (AFP P = 0.02, HCG P = 0.04). Rapid tumour growth rate reflected by short MPDT carries a poor prognosis in patients with NSGCTT treated by BEP chemotherapy.