Fistula-in-ano is a very common surgical condition, caused by anal cryptoglandular inflammation. Most cases are idiopathic. Other causes such as Crohn's disease, trauma and malignancy are well known. Management of fistula-in-ano is largely surgical, especially if the patient is symptomatic. The goal of surgical therapy is sepsis drainage, delineate anatomy and eradicate the fistula while preserving faecal continence. Establishing the aetiology is also crucial as often a combination of specialist medical therapy is required, for example, in Crohn's disease. We report an extremely unusual case of fistula-in-ano on an elderly man with chronic lymphocytic leukaemia (CLL). Histology from the fistula track demonstrated CLL infiltration. This case, not previously reported on PubMed search, illustrates a good example of joint specialist medical (a haematologist) and surgical effort in successfully treating this symptomatic fistula-in-ano.