In a series of 28 neurosurgical cases we tried to assess the value of aspiration biopsy cytology from brain lesions during the operation. Cytological aspirates during craniotomy were compared with simultaneously performed frozen section biopsies. Cytology was correct in 24 out of 28 cases (86%). It is concluded that aspiration cytology permits a safe microscopic diagnosis provided the specimens are cellular and representative. Cases are mentioned where this technique can be very useful and/or the only way to obtain microscopic evidence of a malignant tumour before treatment.