OBJECTIVE: Mannitol is an osmotic diuretic agent and reduces intracranial pressure. The most serious side effect of mannitol is kidney dysfunction. In this study, renal functions in acute stroke patients treated with mannitol were evaluated. METHODS: One hundred and twenty-two patients followed in the neurology intensive care unit with the diagnosis of stroke and treated with fractionated mannitol for 5 days were evaluated retrospectively. Ninety-six patients had ischemic and 26 had hemorrhagic stroke. Mean age was 69.9 ± 11.8 (18-91) years. Serum urea, creatinine and electrolyte levels measured before and on the second, third, fourth, fifth and tenth days of treatment were compared statistically with paired sample t test. RESULTS: The average urea and creatinine levels on the second, third, fourth and fifth days of treatment were significantly higher than the baseline (p< 0.05). On the other hand, mannitol treatment did not change average sodium, potassium and chlorine levels. The creatinine levels had returned to the normal range on the tenth day of treatment, but the urea levels, although decreased, did not fall to the normal range. CONCLUSION: Our results support the view that close monitoring of renal function is necessary in patients treated with mannitol.