A fair % of patients with a clinical history of nickel allergy show negative patch test results. To improve the response rate to NiSO4 5% pet, patch tests, a testing procedure utilizing pre-treatment of I he lest area by a 24-h application of sodium lauryl sulfate (SLS) was introduced 46 women with a clinical history of nickel sensitivity who exhibited negative reactions to nickel sulfate 5% pet, patch tests. were studied, Patients underwent d patch tests on adjacent sites on the volar surface of the forcarms. 4 patch tests were performed with a 72-h application of 40 mg nickel sulfate 5% pet. While I of these patch tests served as control. 3 test areas underwent 24-h pretreatment with 40 μl SLS. 1 with 0.1% and 2 with 0.5% solution. To evaluate differences in the reactivity to SLS plus nickel sulfate related to the site on the forearm, 0.5% SLS pre-treatment was performed both on a proximal and on a distal lest site. At the 72-h evaluation. 19 subjects out of 46 showed positive reaction to nickel sulfate 5%. At skin sites pre-t railed with SLS. Whereas 23 patients reacted positively at 0.5% SLS pre-treated ureas. Echographic values of skin thickness and of hypo-echogeme dermal areas al positive pre-treated nickel lest. Next higher than al control Jest areas, confirming the clinical evidence of an increased response to NiSO4 after SLS pre-treatment. The inflammatory reaction, is evaluated clinically and echographically, was much higher al distal skin areas (0.l% SLS and distal (0.5%.) SLS than at proximal 0.5% SLS ones.