BACKGROUND Malignant transformation remains a rare, under-recognized and ominous, complication of leg ulcers, although its exact prevalence is unknown. METHODS This retrospective French study included cases of chronic ulcers of vascular origin complicated by histologically proven carcinomas. For squamous cell carcinomas (SCC), the duration of the ulcer had to be longer than 3 years. For basal cell carcinomas (BCC), a negative previous biopsy of the ulcer was considered. RESULTS Eighty patients, accounting for 85 tumours, were included, with a female : male ratio of 2.5 : 1 and a mean age of 75 years. Eighty-eight percent of the ulcers were of venous origin and their mean duration was 27.5 years. Five patients developed bilateral cancers. Clinical findings included abnormal granulation tissue in 76% of cases, absence of healing in 14% and unusual extension in 6%. Histologically, 83/85 (98%) of tumours were SCC, among which 82% were very well or well differentiated and 18% moderately or poorly differentiated. The two remaining cases were BCC. The overall death rate was 32%; it was higher when lymph-node (66%) or visceral metastases (83%) were present. Leg amputation was performed in 29/51 (57%) of patients, irrespective of the degree of histological differentiation. For well-differentiated (grade I) and localized (stage Ia) SCC, simple surgical excision was preferred to amputation. CONCLUSIONS Malignant transformation of chronic leg ulcers of vascular origin is mainly encountered in elderly patients and manifests as an abnormally vegetating lesion, which may be occasionally bilateral. Malignant transformation usually occurs towards well-differentiated SCC and only exceptionally towards BCC. The high death rate, especially in metastatic cases, is at least partly due to delay in diagnosis. Surgery remains the treatment of choice. Leg amputation should be considered in the most extensive cases.