Three cases and a pathogenic discussion. We report three cases of clinically typical pyoderma gangrenosum (PG) remarkable for the presence, at histology, of inflammatory vascular lesions of varying intensity. Case No. 1. A 15-year old boy without previous pathology was admitted for necrotic cutaneous lesions typical of PG on both legs. Biopsy in the peri-ulcerous rim showed cellular infiltration of the dermis, principally by granulocytes, associated with granulocytic vasculitis with fibrinoid necrosis (fig. 1 and 2). Laboratory examinations only showed a decrease of CH 50, and attempts at making an aetiological diagnosis of this case of PG met with failure. PG regressed under treatment with disulone combined with topical corticosteroid therapy. Case No. 2. A 62-year old woman with no previous severe disease was admitted for PG of her right leg at the ulcero-necrotic stage (fig. 3). Biopsy in the PG rim showed infiltration of the dermis, predominantly by granulocytes, associated with granulocytic vasculitis and fibrinoid necrosis of the walls of small vessels (fig. 4). Investigations in search of a cause revealed benign thyroid gland hyperplasia. PG was cured after a 5-week oral treatment with minocycline. Case No. 3. This 63-year old woman with arterial hypertension was admitted for PG of both legs. Some lesions at a very early stage were bullous (fig. 5). Biopsy in the peripheral swelling showed polymorphous cellular infiltration of the dermis with a predominance of granulocytes. These cells infiltrated the vessels the walls of which were turgid but without fibrinoid necrosis (fig. 6).(ABSTRACT TRUNCATED AT 250 WORDS)