The aim of this study was to investigate the efficacy and adverse effects of methotrexate (MTX) in the treatment of severe chronic asthma in 12 patients with severe asthma requiring continuous treatment with oral steroids at the Outpatient Department of Helsinki University Central Hospital. The study was a randomised, double-blind placebo-controlled trial of methotrexate treatment 15 mg weekly on a crossover basis over 24 weeks. During the 2 weeks baseline phase the mean dose of oral steroids administered was 10.9 (3.2-28) mg.day-1, and the mean dose of inhaled steroids administered was 2.3 (1.6-3.2) mg budesonide or beclomethasone. The average dose of oral steroids administered was 12.8 mg.day-1 during the last 2 placebo weeks but only 7.9 mg.day-1 during the last 2 weeks with MTX treatment. The reduction in daily dose of oral steroids was 38%, while daily bronchodilator use was reduced by 22%. During MTX treatment the patients experienced significantly less wheezing, dyspnoea and coughing. Nine out of 12 patients reported better asthma control during MTX treatment. The peak expiratory flow rate (PEF) 1-s forced expiratory volume (FEV1) values did not differ between MTX and placebo treatments. There was no statistical correlation between serum MTX concentration and clinical improvement. No serious adverse effects of MTX were found during the study. It was concluded that low-dose MTX may be beneficial for severe chronic asthma and that this therapy is well tolerated by patients.