Eight cases of adenoid cystic carcinoma were reviewed to determine whether and how the histological features of the tumour vary with the progress of the disease. The tumours were classified by their histological patterns as tubular, cribriform, trabecular or solid. The relative amount of each pattern seen in routine light microscopic sections was calculated histomorphometrically and compared in the primary tumour and recurrent and/or metastatic lesions in the same case. In the early stage, the tubular pattern predominated. Later, the highest percentage shifted to the cribriform, then to the trabecular and finally, in the late stage, to the solid pattern. There was no reverse direction of histological transformation from the solid to the tubular pattern. These results may help to explain previous reports that the tubular pattern usually represents a favourable prognosis, the solid pattern a poor prognosis and the cribriform pattern an intermediate prognosis.