Twenty six patients with pleural mesothelioma of UICC stage I-IV excluding M1 disease (46% of whom had stage I disease and 38% stage III disease) were treated intravenously with high dose MTX (3 g) and calcium folinate rescue three times at intervals of 2 weeks and three times at intervals of 3 weeks. Natural interferon (IFN)-alpha (3 MIU days 2-10) and recombinant IFN-gamma1b (50 microg m(-2) on days 2, 6 and 10) were injected subcutaneously after each MTX dose. At the end of MTX treatment the IFNs were continued as maintenance therapy until disease progression. Seven partial responses were observed among 24 patients evaluable for response (response rate 29%, 95% confidence interval 13-51%). Median duration of response was 10 months (range 3-24 months). Median survival was 17 months and 1-year and 2-year survival rates 62% and 31% respectively. The toxicity of the chemo-immunotherapy was acceptable. Treatment was stopped in one patient who developed grade IV neurological toxicity. MTX dose reductions were rare (two patients with grade 1-2 renal toxicity). The combination of high dose MTX and IFN-alpha and IFN-gamma is active against malignant pleural mesothelioma and well-tolerated. The survival rates are encouraging.