Abstract Twenty-four patients with Crohn's disease of the intestine have been treated with azathioprine over periods up to 21 months, seventeen patients with the drug alone, seven in conjunction with surgery. The results have been encouraging in the initial disease (especially colitis), recurrent disease, in the treatment of anal and abdominal fistulæ, and as maintenance therapy in conjunction with surgery to prevent recurrence of the disease and of abdominal fistulæ. Azathioprine alone has proved ineffective in the presence of a palpable fixed inflammatory mass if this is greater than might be expected from the thickened wall of the involved intestine alone and when radiological evidence has demonstrated considerable epithelial loss. Resection is then required, followed by azathioprine. It is not yet clear when azathioprine therapy can be withdrawn.