OBJECTIVE: To analyze and compare the clinical characteristics of mannitol-induced acute renal function impairment in treatment of patients of different ages suffering from subarachnoid hemorrhage (SAH) for the first time. METHODS: This study was a review of 1 361 patients with available hospital records admitted to the division of neurology due to SAH from February 1989 to December 2005. Among them, 94 patients had acute renal function impairment as a result of mannitol administration. Of these patients, 35 patients were middle-aged (<60 years old) and 59 patients of old age (>60 years old). The study included the time of appearance of renal function impairment after mannitol medication, its prognosis, the administration of nephrotoxic drugs, and the dose and duration of mannitol therapy. RESULTS: In old age group, abnormal renal function and urine routine appeared in 5 days (median), and 4 days (median) respectively, and acute renal failure (ARF) was diagnosed in 5 days, and the incidence rate of ARF was 20.3% after treatment with mannitol. The respective event appeared 7 days, 11 days, 9 days and 2.8% (P<0.05 or P<0.01) respectively, in middle-aged group. The data indicated the elderly patients had poorer tolerance to mannitol, with earlier occurrence of mannitol nephrosis, poorer outcome of kidney impairment and worse prognosis. Forty-three patients (72.9%) were treated with katlex (with a median of total dosage of 400 mg) in old-aged group, 35 patients (100.0%) were treated with katlex (with a median of total dosage of 800 mg) in middle-aged group, and there was a significant difference between two groups (P<0.01). The data indicated the incidence of mannitol nephrosis was lower in patients treated with large dosage of mannitol and katlex. The mortality was 3.4% (2/59 cases) and 0% (0/35 cases) in old-aged and middle-aged groups respectively (P=0.528). CONCLUSION: The elderly patients have higher rate of mannitol-induced impairment of renal function after SAH than middle-aged patients. The data emphasize that proper combination therapy of mannitol with katlex is an effective measure in preventing renal failure.