Chronic arsenic toxicity (CAT) manifests predominantly as cutaneous lesions in the form of melanosis, keratosis and neoplastic changes. We have studied skin biopsies from 42 patients of CAT. Histological study of H/E stained sections showed--hyperkeratosis in 13, parakeratosis in 13, acanthosis in 12, papillomatosis in 24, elongation of reteridges in 21, increased basal pigmentation in 27 and dysplastic changes in 8 cases. Squamous cell carcinoma was present in 2, basisquamous in 1 and basal cell carcinoma in 1 case. Changes of skin lesions after drug DMSA and DMPS therapy compared to placebo were studied. The result was inconclusive. Proliferative activity of skin lesions in CAT were studied by AgNOR stain to assess the biological behaviour of the lesions. AgNOR score showed--normal control 1.08, benign changes (e.g. Hyperkeratosis, parakeratosis, acanthosis, papillomatosis etc.) without dysplasia--1.35, mild to moderate dysplasia--1.735, severe dysplasia--3.0 and carcinoma--3.56. Thus, AgNOR score gives some idea on the biological behaviour of CAT lesions. It is suggested that AgNOR staining should be done regularly along with H&E staining for proper assessment of the cases.