An 18-year-old male presented with a 2-week history of rapid progressive dyspnoea and dry cough due to a large mediastinal mass with compression of the trachea. Based on the raised serum values of the tumour markers chorionogonadotrophin and alphafoetoprotein the diagnosis of germ-cell tumour was made. Because of the severity of his symptoms chemotherapy with bleomycin, etoposide and cisplatin was begun on the same day and before the results of the histology investigations were known. The next day the symptoms were diminished and after completing four courses of chemotherapy there was complete remission. The differential diagnosis of a rapid progressive mediastinal mass is limited and mainly relates to malignant lymphoma and germ-cell tumours. In emergency situations if tumour markers are raised then anti-tumour therapy may be begun before any histological confirmation is available.