Thermography can be used as a method of diagnosing breast masses. We report our results of its use in the differential diagnosis of fibroadenoma from phylloides tumours (n = 47 and 20 respectively). Thermographic resolution (Th) and the difference in temperature between the tumour and a similar zone in the contralateral breast (delta 2) were compared. Thermograms were class Th1 (with a similar thermal pattern in both breasts without hypervascularization or hot points) and Th2 (with hypervascularization or a hot area with a thermal difference with the same area in the opposite breast (delta 2) of less than 2 degrees C) in most (95.7 per cent) of the patients with fibroadenoma and were class Th5 (having one or more pathological sign) in 85 per cent of the patients with phylloides tumours. Patients with phylloides tumours had a mean delta 2 of 2.99 degrees C whereas most of the patients with a fibroadenoma showed no difference in temperature. Their mean delta 2 was 0.2 degrees C (P less than 0.0005). We conclude that thermography helps in differential diagnosis between a fibroadenoma and a phylloides tumour.