The detection of an occult tumour in a patient with a suspected paraneoplastic neurological disorder (PND) may be difficult because of the limitations of conventional imaging techniques. [(18)F]fluoro-2-deoxyglucose-PET (FDG-PET) can visualize a small tumour anywhere within the body. We retrospectively reviewed the case notes of 43 unselected patients with suspected PND referred for FDG-PET scanning to determine how useful this technique was when conventional imaging was negative. All patients had undergone standard radiological investigations and bronchoscopy (where appropriate) prior to PET scanning. There were discrete areas of hypermetabolism suggestive of malignancy (positive) in 16 patients (37%). A tissue diagnosis of cancer was subsequently made in seven patients (two at post-mortem), further radiological studies were suggestive of cancer in one patient, one patient subsequently presented with a metastatic deposit which was biopsied, and four patients died shortly afterwards without a post-mortem. In three patients, subsequent investigations were negative for cancer. Serum anti-neuronal antibodies were present in 43% and CSF oligoclonal bands were present in 46% of patients with positive PET scans compared with 16 and 26%, respectively, in PET-negative patients, but this was not significant. Only one patient with a negative scan has been diagnosed subsequently as having malignancy on prolonged follow-up. These findings confirm that FDG-PET scanning is a useful technique in the detection of small tumours in patients with suspected PND. False positives and false negatives do occur, but at a sufficiently low frequency to justify the clinical usefulness of this technique.