Primary tumors of the larynx and hypopharynx are preferably treated with high-dose radiation therapy. In these patients, it may be difficult to distinguish recurrent disease from post-treatment reactions. The aim of the present study was to assess the value of fluorine-18 fluorodeoxyglucose positron emission tomography (FDG-PET) in the detection of local relapses of laryngeal or hypopharyngeal carcinoma after radiotherapy using a dual-head PET camera. Forty-eight patients (43 male, 5 female; mean age +/-SD, 61+/-9.5 years) with suspected recurrent laryngeal or hypopharyngeal cancer were prospectively studied. The mean interval between initial treatment and suspicion of recurrent disease was 14.6 months (range: 3-100 months). FDG dual-head PET was followed by endoscopy with or without biopsy under general anaesthesia within a period of 2 months in all patients. The mean period of follow-up after FDG dual-head PET was 13.7 months. In 19 out of 31 patients with focally increased uptake, tumour recurrence (mean diameter: 2.4 cm; range 0.4-6.5 cm) was found at initial endoscopy. In five patients recurrence was found during follow-up with a mean interval of 6.6 months. Seven patients had a false-positive study due to benign lesions or swallowing artefacts. In none of the patients with a normal PET study was tumour recurrence found during follow-up. The sensitivity and specificity of FDG dual-head PET were 100% and 71%, respectively. It is concluded that FDG dual-head PET is highly sensitive for the detection of local recurrence of laryngeal and hypopharyngeal carcinoma after radiotherapy. Some lesions were detected with a mean interval of 6.6 months before histological confirmation. In patients suspected of having recurrent laryngeal or hypopharyngeal cancer in whom FDG-PET is negative, endoscopy may be omitted for at least 6 months and possibly for up to 1 year.