Arsenicosis is chronic subclinical or clinical toxicity due to high level of arsenic in body. Diagnosis of arsenicosis was derived by chronological establishment of facts: (a) arsenic as the cause of malady, (b) drinking water (tubewell water) as the vehicle of arsenic, (c) soil as the source of arsenic, (d) mechanism of leaching of arsenic from soil, and (e) cause of prevalence in particular areas of the country. Arsenicosis has been classified by the author into 4 stages, 7 grades and 20 subgrades. Stage I is pre-clinical or grade 0. While clinical features were not found at this stage, high level of arsenic metabolites was observed in urine. As disease progressed to stable phase of grade 0, high level of arsenic was also found in nails, hair, and skin scales. Stage II or clinical stage is divided into 4 grades, (1) Melanosis, (2) Spotted keratosis in palms/soles, (3) Diffuse keratosis in palms/soles, and (4) Dorsal keratosis. Clinical complications are grouped in stage III and grade 5. Malignancy is considered in stage IV and grade 6. There is a concern of both underdiagnosis and overdiagnosis. Therefore, cases of arsenicosis should be cautiously evaluated. Melanosis was the earliest cutaneous sign of clinical arsenicosis. Mild cases of melanosis could only be revealed by a thorough comparison with normal palms. Similarly mild cases of keratosis might not be visible and could only be revealed by careful palpation of palms and soles. Combination of melanosis and keratosis in adults indicated clinical diagnosis of arsenical dermatosis (ASD) that should be confirmed by showing high arsenic concentration in body tissues e.g., nails, hair, skin scales. Isolated melanosis or keratosis in newborn or children below 2 years almost negated the diagnosis of arsenicosis. Genetic melanosis or keratosis is often present since birth. Isolated melanosis or keratosis in adults should be differentiated from non-arsenical dermatosis and proven by absence of high arsenic level in nails and hair. Non arsenical causes of diffuse melanosis, spotted melanosis or leucomelanosis and localized or generalized keratosis can be clinically differentiated from arsenicosis by absence of pigmentation and keratosis in palms/soles.