Although chronic subdural hematoma (CSDH) is a well-known entity, its recurrence rate has remained uncertain. There is little knowledge concerning whether the results of radiological imaging can be used to predict CSDH recurrence or whether surgical methods can influence this rate. The first aim of this study is to evaluate the relationship between the recurrence rate of CSDHs and their appearance on preoperative magnetic resonance (MR) or computerized tomography images. The second aim is to evaluate by means of a prospective randomized method the usefulness of closed-system drainage. From January 1988 through June 1996, the authors surgically treated 257 consecutive adult patients with CSDHs. Data obtained in 199 patients who were evaluated preoperatively by MR imaging were analyzed. Thirty-one of these patients underwent bilateral operations and thus 230 operative sites of CSDH were included in the analyses. The cases of CSDH were separated into high- and nonhigh-intensity groups on the basis of the appearance on T1-weighted MR images. From July 1992 to June 1996, the authors conducted a prospective randomized study on the recurrence rate of CSDH in patients undergoing burr-hole irrigation with or without closed system drainage. The recurrence rate of 3.4% in the high-intensity group was significantly lower than the 11.6% rate found in the nonhigh-intensity group (p < 0.05). The recurrence rates following irrigation with and without closed system drainage were significantly different (p < 0.025): 3.1% with closed system drainage and 17% following burr-hole irrigation alone. The surgical procedures were correlated with the MR findings. In the high-intensity group, 1.1% of CSDHs recurred in patients in whom closed system drainage was used and 11.1% in patients without closed system drainage. In the nonhigh-intensity group, 8.1% of CSDHs recurred in patients in whom drainage was used and 23.1% in patients without closed system drainage. Magnetic resonance T1-weighted imaging was useful in predicting the propensity of CSDHs to recur. Closed system drainage significantly reduced the recurrence rate of CSDHs regardless of MR findings.