Flexor tenosynovitis is an aggressive closed-space infection of the digital flexor tendon sheaths of the hand. We present a case of pyogenic flexor tenosynovitis in an immunocompromised patient and discuss the importance of early diagnosis and referral to a specialist hand surgery unit. A 61-year-old man visited his general practitioner because of swelling and tenderness of his left index finger. The patient was discharged on oral antibiotics but returned 4 days after because of deterioration of his symptoms and was referred to a plastic surgery unit. A diagnosis of flexor tenosynovitis was made and the patient required multiple debridements in theatre, resulting in the amputation of the infected finger. Pyogenic flexor tenosynovitis is a relatively common but often misdiagnosed hand infection. Patients with suspected flexor tenosynovitis should be referred and treated early to avoid significant morbidity, especially when risk factors for poor prognosis are present.