Peripheral neuropathies are among the most common neurological diseases and have numerous causes, including diabetes, alcohol, hereditary, toxic, metabolic, infectious, inflammatory, ischaemic and paraneoplastic. Often, however, no definitive cause is identified and the condition is termed idiopathic neuropathy. Here we describe a patient who was initially diagnosed with idiopathic sciatic neuropathy but who was eventually diagnosed with prostate cancer. This is an uncommon manifestation of prostate cancer, and the diagnostic was difficult because prostate-specific antigen (PSA) was normal and the positron emission tomography scan negative. Changes in PSA should always raise the suspicion of prostate cancer, just as idiopathic progressive neuropathy should always raise the suspicion of an underlying malignancy, even when standard diagnostics fail to explain the patient's symptoms.