With a view of elucidating factors influencing mortality in patients with thoracic empyema, which varies widely (10-50%) in reported case series, a retrospective analysis was made. The series comprised 259 patients treated for thoracic empyema at Bispebjerg Hospital in the period 1965-1980. The mortality was 33% in the total case series, 61% when the underlying pathology was malignant and 25% when it was benign. As malignancy was apparently cured in only 17% of the cases, the investigation was focused on the 200 patients with benign conditions underlying the empyema. The mortality then ranged from nil in spontaneous pneumothorax and thoracic trauma to 50% in lung abscess. Most of the patients with empyema were elderly, but there was no clear difference in mortality between younger and older groups. Concomitant, other disease was present in 80% of the patients who died, but in only 40% of the survivors. Mortality showed no significant difference in relation to primary treatment. Staphylococcus aureus was statistically predominant among the fatal cases. It is concluded that empyema occurring, as in the present study, mainly in elderly and enfeebled patients, is a serious complication with high mortality.