23 cases of supratentorial extracerebral cysts of the Sylvian fissure in children and adult patients are reported. There were 21 male and 2 female patients and the medial age was 15 years (range 2--60 years). The cysts were located in the left middle cranial fossa in 18 cases, in two cases there were bilateral cysts and three patients only had right-sided lesions. In 14 patients the presence of the cyst was uncomplicated. In two of these patients the cyst was detected only incidentally. Four patients had subdural haematomas in addition to the cyst. Two patients had subdural haematomas and were hydrocephalic as well. Three patients had large cysts and hydrocephalus. We did not encounter intracystic bleeding. Operation on the cysts by uncapping them was done in four cases. In all four instances histological examination of the cyst wall revealed that it was composed of arachnoid. Uncapping of the cyst was not followed by its diminution in any of the four patients. It seems that uncapping of such cysts is not a helpful treatment and that surgical treatment in our cases should have been restricted to complications such as subdural haematomas and hydrocephalus. A search for better operative methods for the treatment of large extracerebral supratentorial cysts of the middle cranial fossa should be undertaken.