A 15-year-old girl with her first relapse of ulcerative colitis was treated with salicylazosulfapyridine, steroids, metronidazole and parenteral nutrition for 4 weeks without significant improvement. Her faecal specimens were negative for established intestinal pathogens, but strains of Escherichia coli producing Shiga-like toxin and expressing binding of fibronectin and collagen were repeatedly isolated. Neutralizing antibodies to Shiga-like toxin were detected in serum samples. Improvement was achieved during treatment with mecillinam to which the E. coli strains were sensitive.