In 29 patients with metastasising testicular tumours serial determination of HCG was performed for monitoring cytostatic chemotherapy. Clinical remissions and stable disease were accompanied by normalisation of elevated HCG values in 85% and 80% of all cases, respectively. Despite the normalisation of HCG levels, no complete disappearance of clinically detectable tumour lesions could be seen in the herein presented cases. Seventeen percent of the patients showed normal HCG levels, although they had progressive disease. Recurrent disease was associated with preceeding increase of HCG in only 33% of the cases. Because of the frequency of mixed tumour tissues in teratomas, additional methods for monitoring therapy should be used.