The objectives of this study were to evaluate the efficacy of a sulphadiazine desensitization protocol to treat patients with AIDS and cerebral toxoplasmosis (CT) and known sulphonamide allergy, to ensure that an adequate dose of sulphadiazine (2–4 g/day) was achieved rapidly (within 4–5 days), and to assess the effect of concurrent corticosteroid (CS) administration on the success rate of the regimen. Sixteen patients with CT and a past history or current manifestations of sulphonamide allergy were desensitized to sulphadiazine from October 1988 to December 1989. The protocol employed the oral administration of gradually increasing increments of sulphadiazine 3-hourly over 5 days. Success was defined as tolerance of 2–4 g oral sulphadiazine per day for at least 7 days until death or the present time without any allergic reactions. Our success rated overall was 10 out of 16 patients (62%). Seven patients achieved a final dose of 4 g/day and three a dose of 2 g/day. Concurrent CS administration did not appear to affect the outcome in the small number of patients studied. Our sulphadiazine regimen rapidly, successfully and safely desensitized patients with CT and sulphonamide allergy, allowing the optimal first-line treatment to continue. The aetiology of allergy in HIV-infected patients and the mechanisms by which desensitization works are unknown.