The uptake of iodine-123 alpha-methyl-l-tyrosine (IMT) in the primary tumours and metastatic lymph nodes of squamous cell carcinoma of the head and neck was examined with single-photon emission tomography (SPET). Eleven patients with biopsy-proven carcinomas were studied prior to any therapeutic action. The evaluation of cervical lymph node involvement was based on the findings of physical examination, computed tomography and magnetic resonance imaging, and in six patients on the histological data relating to tissue samples obtained by fine-needle lymph node aspiration or surgical intervention. SPET imaging was performed 10 min after the injection of 130-170 MBq IMT using a triple-head gamma camera equipped with medium-energy collimators. High-quality IMT SPET depicted the primary tumour in 10 of 11 patients (sensitivity: 91%). Tumours located in the larynx were visualized more clearly than those located in the mouth or oropharynx. The mean tumour-to-background ratio was 2.35 (range: 1.6-3.1) for laryngeal tumours and 1.67 (range: 1.2-2.2) for mouth and oropharyngeal tumours. Metastatic cervical lymph nodes were involved to various degrees in 8 of the 11 patients. Among these eight patients there were 16 sites, nine of which were detected by IMT SPET (sensitivity: 56%). If the IMT SPET findings were recorded per side of the neck, the sensitivity was 64%. Five of the seven missed metastatic lymph nodes were smaller than 15 mm. The mean tumour-to-background ratio of the scintigraphically visualized lymph nodes was 1.81+/-0.51 (range: 1.39-2.77). Asymmetric physiological submandibular salivary gland IMT uptake led to false-positive lymph node assessment in three patients. This study indicates the potential use of IMT SPET as a metabolic imaging modality in patients with head and neck squamous cell carcinoma.