We here report the case of a 35-year patient treated for anoperineal Crohn's disease with corticosteroids and then 6-mercaptopurine and azathioprine inducing prolonged remission. Patient's evolution was marked by fast deterioration of general health status. Morphological examination were in favor of severe outbreak of disease. The diagnosis of fistulizing anoperineal Crohn's disease was made (A). The patient was treated with broad-spectrum antibiotics, parenteral feeding, digestive system at rest and defunctioning colostomy allowing clinical improvement. Then he was treated with anti-TNF-alpha Infliximab at a dosage of 5mg/kg (S0, S2, S4 and S8). Patient's evolution was complicated by the gradual onset of a tumor-like anal lesion (B) progressively increasing its volume associated with fast deterioration of general health status. New abdominal MRI allowed to detect anorectal tumoral process, immunohistochimically in favor of infiltrating ano-rectal adenocarcinoma. 4.3% of cases with anorectal adenocarcinoma associated with fistulizing Crohn's disease have been reported. This study aimed to attentively analyse possible malignant transformation of fistulizing Crohn's disease in any patient treated with infliximab and to highlight the role of histological assessment of any anal or perineal lesion before treatment with anti-TNF therapy. Diagnosis is difficult because due to inflammatory status and/or concomitant stenosis; imaging tests, including CT scan and MRI appear to have a low sensitivity in detecting cancer.