Acute SVCO presents a serious diagnostic and therapeutic dilemma to the thoracic surgeon. It is highly desirable to obtain a definitive tissue diagnosis but this requirement must be balanced against the risks inherent in invasive diagnostic procedures in these oft-times critically ill patients. In the past 5 years we have developed an algorithm that has been used successfully in 18 patients presenting with acute SVCO. The decision tree consists of scalene node biopsy, bronchoscopy and mediastinoscopy with categorization into low and high risk groups in the latter. The gender distribution of the 18 patients was 12 male and 6 female with the ages being 58.3 +/- 16.3 and 64.2 +/- 11.2 years respectively. The duration of symptoms range from 1.5 to 12 weeks and averaged 4.5 +/- 3.2 weeks. Two patients had palpable scalene nodes which were positive at biopsy. Bronchoscopy was positive in 5 out of 11 examinations. In 5 instances it was not done. Twelve patients underwent various forms of mediastinal biopsy and one underwent sternotomy. Seven patients requiring mediastinoscopy were judged to be high risk as defined by severe airway and vascular obstruction. Perioperative difficulties occurred in two patients due to cardiorespiratory factors associated with the obstruction. Both patients were undergoing cervical mediastinoscopy under general anesthesia and no difficulties were encountered in high risk patients when local anesthesia was used to perform anterior mediastinotomy. Five patients that were considered low risk all underwent uncomplicated procedures under general anesthesia. Thirteen patients proved to have bronchogenic carcinoma and a specific tissue diagnosis was obtained in 12. The undetermined diagnosis was in a patient who had prior radiotherapy.(ABSTRACT TRUNCATED AT 250 WORDS)