Malignant external otitis (MEO) is a severe infectious disorder usually caused by Pseudomonas aeruginosa, which most frequently affects diabetic patients. Due to its rarity, the diagnosis of MEO is often not made promptly. Extension into deeper structures or chronic osteomyelitis may occur without signs of infection on local clinical examination. Of the imaging techniques, magnetic resonance imaging provides a fairly adequate picture of the spread of the disease, but, as with computed tomography (CT) scanning and bone scintigraphy, the images remain unchanged for a long time after disease regression. The objective of this study was to establish whether quantitative gallium-67 single-photon emission tomography (SPET) represents an accurate method for the assessment of infection and, moreover, for the monitoring of therapeutic effect. Eight patients (five males, three females) with the clinical diagnosis of MEO were studied. In three patients antibiotic treatment was prolonged for several weeks because visual analysis of gallium scintigraphy still showed slightly increased uptake in the affected area on the first follow-up scan. In one patient, it was decided to stop antibiotic treatment despite a slight increase in uptake on the second follow-up scan. Lesion to non-lesion (L/NL) ratios obtained from 67Ga SPET images at initial diagnosis and during follow-up were assessed in correlation with clinical and biochemical data and with the results of CT scans. In addition to a raised erythrocyte sedimentation rate (ESR), all patients showed increased uptake on the affected side, with L/NL ratios ranging from 1.4 to 3.6 at the time of diagnosis. CT scans failed to demonstrate abnormalities in four patients. Including four scans demonstrating slightly increased uptake in the affected area, L/NL ratios after 6-8 weeks of antibiotic treatment were 1.0+/-0.1. Despite a persistently elevated ESR in the majority of patients, none of them demonstrated local recurrence or complications during follow-up. In all patients, leucocyte count was within the normal range throughout the course. No relation was found between the slightly increased uptake on the follow-up scans and surgical treatment. It is concluded that in addition to the visual analysis of 67Ga SPET imaging, L/NL ratios should be calculated for a more accurate assessment of disease activity in MEO. Despite visually slightly increased uptake, L/NL ratios of 1.0+/-0.1 during follow-up are highly indicative of complete recovery, regardless of ESR values or leucocytosis. CT scans are of little value for diagnosis or for monitoring of therapeutic effect.