We studied 28 patients with fixed drug eruption (FDE) caused by sulfonamide antibiotics to investigate cross-reactivity between sulfonamide derivatives and p-amino compounds and to explore the usefulness of patch testing, as an alternative to controlled oral challenge testing (COCT), in diagnosis within this clinical area. COCT with sulfamethoxazole (SMX), sulfadiazine (SDZ), sulfamethizole (SMZ), furosemide (FU), procaine (PRO) and glipizide (GPZ) was performed. Patch testing (PT) with SMX and SDZ was carried out. In all patients, the diagnosis of FDE was confirmed by positive COCT and allergy to trimethoprim ruled out by COCT. 42.8 and 31.8% of the SMX-induced FDE patients reacted to SMZ and SDZ, respectively. All patients (n = 28) tolerated FU, PRO and GPZ. COCT performed with the 3 sulfonamide antibiotics in 12 patients was positive in 2 subjects with the 3 drugs, in 2 patients only with SMX and SMZ and in the remaining 8, SMX was the only causative drug. PT was positive in 5 of 25 patients positive on COCT. The probability of obtaining a positive PT was higher among patients who had a residual lesion than that among those who lacked this. Cross-reactivity between different sulfonamide antibiotics is thus variable, being most likely between SMX and SMZ. We have found no cross-reactivity between sulfonamide antibiotics and other sulfonamide derivatives or p-amino drugs in FDE. PT is a useful tool in the diagnosis of FDE, especially if there are residual lesions, because it avoided the need for COCT in 20% of patients.