Background: We have analyzed the records of our own hospitalized cases of bilateral chronic subdural hematoma (cSDH) to reveal the prognosis. Methods: Ninety-eight cases of cSDH were operated at our hospital over a 6-year period, in which 14 cases were classified as being bilateral. Among these 14 cases, 6 cases showed a rapid and aggressive clinical course. Therefore, complicated risk factors, the initial data on coagulofibrinolytic examination, magnetic resonance imaging appearance, and prognosis were analyzed. Results: Of the 6 cases, 5 showed a rapid aggravation as they awaited surgery. The period of the aggravation since the initial diagnosis harboring cSDH was 19 to 54 hours. One case was at first neurologically free from any disturbance but 17 hours later experienced a generalized seizure. All 6 cases experienced consciousness disturbance. In addition, 3 of them manifested oculomotor palsy. Two cases showed an abnormality of coagulofibrinolytic activity. No significant risk factors were revealed. In 4 cases, T-2-weighted images (T2WIs) revealed the hematoma of a mixed high and low intensity, indicating that the hematoma consisted of both liquid and solid parts of a freshly formed blood clot. In 2 cases, the hematoma showed a low intensity in T-1-weighted image (T1WI), indicating a recent bleeding of a significant amount. Conclusion: The bilateral cases of cSDH should be treated as early as possible with simultaneous decompression of bilateral hematoma pressure, even if the patient shows minimal neurologic deficits. Mixed high and low intensity in T2WI or low intensity in T1WI is the most predictable factor to show rapid aggravation. (c) 2005 Elsevier Inc. All rights reserved.