The cases of 42 patients with malignant superior vena cava (SVC) obstruction were reviewed to evaluate clinical dogmas of prohibitive risk for invasive diagnostic procedures and need for urgent radiotherapy. Thirty-nine had carcinoma (35, bronchogenic and 4, other), and 3 had lymphoma. Lung cancer histology was squamous cell in 11, adenomatous in 10, large cell in 7, and small cell in 7. The SVC obstruction was always symptomatic, usually causing facial or cervical swelling, but there was no instance of SVC obstruction causing life-threatening problems such as cerebral or laryngeal edema. Twenty-two patients underwent bronchoscopy (11 flexible and 11 rigid) prior to radiotherapy without respiratory complications, and diagnostic tissue was obtained in 8. Also prior to radiotherapy, 29 invasive diagnostic procedures were performed: thoracotomy (1), mediastinotomy or mediastinoscopy (11), supraclavicular or scalene node biopsy (15), and percutaneous lung needle biopsy (2). Neither excessive blood loss nor serious complications occurred, and diagnostic tissue was obtained in 22 patients who received individualized therapy. Eight patients had urgent radiotherapy, which delayed diagnosis and specific therapy for two weeks to 6 months. For the 33 patients who underwent radiotherapy after development of the SVC obstruction, the obstruction clinically resolved spontaneously within fourteen days, independently of whether radiotherapy was begun immediately or was delayed. Median survival was 5.0 months and was not influenced by the dose or timing (early or late) of radiotherapy. We reached the following conclusions. First, although a grim prognostic sign, SVC obstruction is rarely life-threatening and typically resolves spontaneously, probably by development of venous collaterals.(ABSTRACT TRUNCATED AT 250 WORDS)