Background and Objective: The allergic contact reaction is a model reaction for studying the cell-mediated immune system of the skin. In this study we use a noninvasive method, electrical impedance (IMP), to compare nickel (Ni) allergic contact reactions with an irritant contact reaction induced by sodium lauryl sulfate, which has already been carefully evaluated with this method. For this purpose, we included only Ni- and sodium lauryl sulfate-induced reactions of very similar appearance. Methods: Various concentrations of Ni sulfate in distilled water were applied on the volar aspect of the forearms of 33 adult women who were allergic to Ni. Assessments were made using visual scoring, a new IMP technique, and transepidermal water loss 3 and 7 days later. Results: In the 19 patients who completed the study, 3 of the 4 impedance indices were significantly lower at the sodium lauryl sulfate sites than at the Ni sites on day 3; ie, the mean magnitude (MIX) (P ≤ .001), imaginary part (IMAX) (P ≤ .001), and real part (RIX) indices (P ≤ .01). Unlike the irritant reactions, no significant increases in transepidermal water loss occurred in the allergic contact reactions. This may be because, in reactions of the studied magnitude, an allergic contact reaction does not significantly affect the epidermis because the inflammatory process is located deeper in the dermis than an irritant reaction. Conclusion: This study suggests that IMP, as used herein, is suitable for distinguishing between contact reactions of allergic and irritant nature. Although pathophysiologic events in the tissue studied significantly modify impedance patterns, little is known about how to interpret the structural and chemical changes underlying these patterns. Studies are needed to determine the relation between anatomic or pathophysiologic parameters, and the findings using IMP and other established methods, such as chemical extraction and histopathology. (J Am Acad Dermatol 2003;48:394-400.)