Skin blood flow in allergic contact reactions and cross‐sensitivity were evaluated using laser Doppler flowmetry (LDF) to study the dose‐response relationships in phases of induction and challenge in guinea pigs. Guinea pigs were sensitized with different doses of 1‐chioro‐2,4‐dinitrobenzene (DNCB) and challenged with different doses of DNCB and 2,4‐dinitrobenzene sulfonic sodium salt (DNBS). The skin reactions were evaluated by LDF and visual reading score. The results indicated that there were dose‐response relationships between the doses of DNCB and LDF measure‐men is in both phases of induction and challenge, that there was a cross‐reaction between DNCB and DNBS. and that the reactions at 24 h were greater than that at 48 h after removal of the patches. LDF may discriminate between positive patch test reactions and negative or doubtful reactions, but not between weak positive and strong positive reactions. This is because vascular dilatation and increase of flow already reaches a maximum in weak reactions. The more advanced phases are dominated by oedema formation. This is simply the nature of the inflammatory reaction, rather than a methodological error. The important point is that LDF, can separate positive reactions from negative/uncertain reactions. The results indicated that LDF, as a non‐invasive technique, may objectively und quantitatively evaluate the dose‐response relationships of contact sensi