Percutaneous aspiration of brain tumor cysts via the Ommaya reservoir system. We performed percutaneous aspiration of 21 brain tumor cysts in 20 patients using the Ommaya reservoir system. Ages ranged from 3 to 70 years, median 48. Sixteen were primary tumors (12 anaplastic glioma, 2 craniopharyngioma, 1 oligodendroglioma, 1 brainstem glioma) and 4 were metastatic. Fourteen had the CT appearance of a true cyst and 7 a pseudocyst. We placed 18 catheters through twist drill holes via CT stereotactic guidance and 3 through burr holes via CT guidance and effectively aspirated 3 to 50 ml cyst fluid from 1 to 18 times in each patient. Postaspiration CT showed complete or significant reduction in cyst size in all patients in whom it was performed (18 after initial aspiration and 9 after subsequent aspirations). Asymptomatic intracyst hemorrhage occurred in 2 patients after cyst wall biopsy and catheter placement. There have been no other complications at follow-up of 4 to 114 weeks. In our experience, tumor cyst aspiration by the Ommaya reservoir system is as effective as percutaneous needle aspiration, but after catheter placement aspiration can be performed with minimal technical skill, avoiding repeated CT guidance required for needle aspiration of recurrent deep-seated cysts. 