Objective. Malignant transformation of fistulas has been observed, particularly in perianal fistulas in Crohn's disease (CD) patients. The prevalence of adenocarcinoma in enterocutaneous fistulas and non-CD-related fistulas, however, is unknown. We investigated adenocarcinoma originating from perianal and enterocutaneous fistulas in both CD patients and non-CD patients from nine large, mostly tertiary referral, hospitals in The Netherlands. Methods. Patients suffering from fistulizing disease and either dysplasia or adenocarcinoma between January 1990 and January 2007 were identified using the nationwide automated pathology database (PALGA). Clinical and histopathological data were collected and verified using hospital patient-charts and reported by descriptive statistics. The total CD-population comprised 6058 patients. Results. In a study-period of 17 years, 2324 patients with any fistula were reported in PALGA. In 542 patients, dysplasia or adenocarcinoma was also mentioned. After initial review and additional detailed chart review, 538 patients were excluded, mainly because the adenocarcinoma was not related to the fistula. In the remaining four patients, all suffering from CD, adenocarcinoma originating from the fistula-tract was confirmed. The malignancies developed 25 years (IQR 10--38) after CD diagnosis, and 10 years (IQR 6--22) after fistula diagnosis. Median age at time of adenocarcinoma diagnosis was 48.3 years (IQR 43--58). Only one patient had clinical symptoms indicative for adenocarcinoma. In three other patients, the adenocarcinoma was found coincidently. Conclusions. Adenocarcinoma complicating perianal or enterocutaneous fistula-tracts is a rare finding. Only 4 out of 6058 CD patients developed a fistula-associated adenocarcinoma. We could not identify any malignant transformations in non-CD-related fistulas in our 17 years study-period.