Diagnosis of cement or nickel dermatitis depends on, detailed occupational history, clinical presentation and confirmation by patch testing. Although the patch test is not perfect bioassay, it possesses unique and valuable features. The classic positive patch test reaction, safely interpreted as allergic contact sensitivity consists of erythema, oedema and small closely set vesicles,which extend beyond the borders of the patch. The present study was conducted to throw a light on the patch test from histological, immunological and ultra structural aspects, in order to evaluate its role in diagnosis of contact dermatitis.