The distribution of 3-[123I]iodo-L-alpha-methyltyrosine (123I-3-IMT) in the tumour region of 21 patients with clinically suspected recurrent squamous cell head and neck carcinoma was studied. Single-photon emission tomography (SPET) imaging of the head and neck region was performed 10 min after the injection of 130-170 MBq 123I-3-IMT using a dual-detector gamma camera. Images were interpreted visually and classified as positive or negative for recurrent disease. In addition, target to background ratios (T/B) were measured using semi-automated region of interest analysis. IMT-SPET results were compared with the data derived from clinicopathological follow-up. IMT-SPET detected recurrent disease in 14 of 15 patients (sensitivity 93%). T/B ratios ranged between 1.5 and 2.4 (mean 1.88). One patient with a small tumour (1.2 cm) had a false-negative result. This is attributed to the limited spatial resolution of the SPET system. Five of six patients were correctly diagnosed to be negative for tumour recurrence. T/B ratios ranged between 1.2 and 1.4 (mean 1.30). In one patient IMT-SPET was positive without evidence of recurrence based on clinicopathological follow up. This finding was probably due to uptake into inflammatory tissue. IMT-SPET appears to be a sensitive tool (93%) for the detection of recurrent head and neck squamous cell carcinoma. Further studies with 123I-3-IMT as a metabolic tracer for the detection of head and neck cancer recurrence using SPET are recommended.