In this paper the authors present the first evidence that meningiomas respond to treatment with hydroxyurea. Hydroxyurea was administered as an adjunct chemotherapeutic treatment in patients with recurrent and unresectable meningiomas. Hydroxyurea was used because experimental data demonstrated that it inhibits growth of cultured human meningioma cells and meningioma transplants in nude mice by inducing apoptosis. The authors therefore treated four selected patients with hydroxyurea. All patients had undergone multiple gross resections and all except one received radiotherapy. Three patients with recurrent Grade I meningiomas assessed according to World Health Organization (WHO) guidelines received hydroxyurea because of an increased tumor growth rate, documented by magnetic resonance (MR) imaging, within a 6- or 12-month interval. A fourth patient with a malignant meningioma (WHO Grade III) began a course of treatment with hydroxyurea immediately after his sixth palliative operation without waiting for another relapse to be demonstrated on MR imaging. Because of their location and invasive growth behavior none of the meningiomas could have been removed completely by surgical intervention. All patients received hydroxyurea at a dosage level of 1000 to 1500 mg/day (approximately 20 mg/kg/day). In a man with a large sphenoid wing meningioma invading the right cavernous sinus and the temporal base, the intracranial tumor mass was reduced by 60% during 6 months of treatment. A woman with a large ball-shaped meningioma of the right sphenoid wing invading the cavernous sinus exhibited a 74% decrease of the initial tumor volume in 10 months of treatment with oral hydroxyurea. Serial MR images obtained monthly revealed that the process of size reduction was continuous and proportionate. The shrinkage of the tumor was accompanied by a complete remission of symptomatic trigeminal neuralgia after 2 months and by improved abducent paresis after 5 months. The third patient had a slowly growing meningioma that exhibited a 15% reduction in mass when reassessed after 5 months of hydroxyurea treatment. The fourth patient with the malignant meningioma in the left cerebellopontine angle has had no recurrence for 24 months. Long-term treatment with hydroxyurea may result in full remission of tumors in meningioma patients. The preliminary data indicate that hydroxyurea provides true medical treatment in patients with unresectable and recurrent meningiomas, replacing palliative surgery and radiotherapy in the management of this disease.