In order to assess the value of 20-MHz sonography in the pre-operative diagnosis of malignant melanomas, 54 melanomas were examined. The pre-operative ultrasound scans were compared with corresponding histological sections from the excised tumours. A computer-aided measurement of the tumour thickness (sonometry) and the internal echo density (densitometry) was performed in the ultrasound scans. The melanomas appeared as largely echolucent zones. There was a significant correlation between the tumour thickness measured in the ultrasound scan and those measured in the histological sections (r = 0.938, p less than 0.001). As a rule the tumour thickness determined by sonometry were greater than the histometrically determined values. Subtumoral inflammatory infiltrate and other hypo-echoic structures in the region of the tumour are possible causes of the discrepancies. Some of those hypo-echoic structures can be identified and disregarded in B scan measurement. Even if a definite differential diagnosis is not possible on the basis of the ultrasound scan alone, 20-MHz sonography provides additional information which can be of use in surgical planning.