The aim of this study was to assess the feasibility and effectiveness of intrapleural administration of recombinant human interleukin-2 (rhIL-2) for treatment of malignant pleural effusions. From May 1993 to May 1995, 21 patients with non-small cell lung cancer (NSCLC) and cytologically documented malignant pleural effusion entered the study. After complete needle drainage of the effusion, a dose of 9 x 10(6) international units (IU) rhIL-2 was injected intracavitarily. The drug was injected on the first and second day each week for 3 weeks, in relation to the presence of effusion. Complete response (according to the criteria reported by Paladine st al.) was achieved in 7 of the 21 patients (33%), and partial response in 6 of the 21 patients (29%), with a median duration of 8 months (range 4-10 months). The treatment was well tolerated by all patients. Intracavitary administration of low-dose recombinant human interleukin-2 in malignant pleural effusion due to non-small cell lung cancer is an effective and well tolerated therapeutic strategy.