42 patients with acne vulgaris, clinically resistant to prior therapy with tetracyclines, were evaluated after therapy with sulphamethoxazole-trimethoprim (400 + 80 mg) twice daily. Initially and after 6, 12 and 18 weeks of treatment in each patient the different acne lesions were counted and pus specimens from unhealed pustules were taken for bacteriological analysis. Complete remission or excellent results were obtained in 33 patients (79%) at the end of treatment despite a relative increase of Staphylococcus hominis and Propionibacterium granulosum. These species were more resistant in agar dilution test to the combination sulphamethoxazole-trimethoprim (20:1) than other isolated species.