The Lucio-Latapí's leprosy or diffuse lepromatous leprosy is a clinical variety of lepromatous leprosy first described by Lucio and Alvarado in 1852 and reidentified by Latapí in 1936. It is frequent in Mexico (23%) and in Costa Rica and very rare in other countries. It is characterized by a diffuse infiltration of all the skin which never is transformed into nodule, by a complete alopecia of eyebrows and eyelashes and body hair, by anhydrotic and dysesthesic zones of the skin and by a peculiar type of lepra reaction named Lucio's phenomenon or necrotic erythema which is a vascularitis of vessels especially of the dermohypodermic union and of the hypodermis. Clinically this vascularitis is represented by well-shaped erythematous spots, later becoming necrotic with scabs, ulcerations and scars. Three points of confusion are stressed: the differences between nodules and nudosities, Lucio's leprosy and Lucio's phenomenon and necrotic erythema and necrotic erythema nodosum leprosum. The differences between the pure and primitive form of Lucio's leprosy and the secondary one is also discussed such as the laboratory findings, histopathological data, pronostic and treatment. Lucio's leprosy is considered the most anergic one of the all immunological spectrum of leprosy.