Sonography can be used to localize subcortical or deeply located brain lesions during operation. 22 lesions were found on 19 patients undergoing 20 craniotomies. Our conclusions are the following: A common linear array system, transmitting a frequency of 5 MHz, is suitable and gives high quality pictures, but a small high frequency sector scanner with optimal superficial resolution would appear to be desirable. After craniotomy, sonography may discover additional lesions not identified during previous investigations (CT or angiography). Close observation of the sonographic appearance of a brain lesion can give information complementary to that gathered by preoperative radiology.