Skin blood flow (SBF) in allergic and irritant reactions was determined by laser Doppler flowmetry (LDF) in an attempt to differentiate these reactions in an objective way. 12 subjects with known allergy to nickel were patch tested with nickel sulphate, nickel chloride, and with sodium lauryl sulphate (SLS) 1, 2, 5 and 10%. A positive correlation between the concentration of SLS and SBF was found. Compared with control sites, SBF in weak (+) and strong (++) positive allergic and irritant reactions was increased approximately 5–10 fold 1 and 2 days after application, and decreased towards normal 7 days after application. At day 7, the mean SBF was higher in allergic reactions than in irritant reactions. However, no consistent differences were found. It is concluded that although LDF can be used to quantify the strength of allergic and irritant skin reactions, this method cannot be used solely to separate these 2 types of reactio