The incidence of seizures in patients undergoing burr-hole crainiostomy with closed-system drainage for chronic subdural hematoma (CSDH) is low. The post-operative use of anticonvulsants is, thus, controversial. In this study, we tried to correlate pre-operative computed tomographic (CT) appearance of the CSDH with the need for post-operative seizure prophylaxis. From April 1998 to November 2001, 128 cases of CSDH surgically treated at our hospital were studied. All patients underwent burr-hole craniostomy with closed system drainage. All CSDHs were classified as low-density, isodense, and mixed-density lesions according to CT findings. The incidence of early post-operative seizures (within 3 weeks of surgery) among all patients was 5.4% (7/128). In the subgroups by lesion density, the incidences were 6.2% (1/16) in the low-density group, 2.4% (2/83) in the isodense group, and 13.7% (4/29) in the mixed-density group (all p < 0.05). The mean age among the seven patients (five males and two females) who had seizures was 71 years. The locations of the CSDHs among the 128 patients were the left side of the brain in 53 (41.4%) patients, right side in 45 (35.2%), and bilateral in 30 (23.4%) patients. Among the seven patients who suffered from post-operative seizures, five (71.4%) had left side CSDHs, one (14.2%) had a right side CSDH, and one (14.2%) had bilateral CSDHs. We concluded that the post-operative seizure rate appeared high in the group with mixed-density type lesions on CT, and in those with left unilateral CSDH. We suggest the use of prophylactic anticonvulsants for patients with mixed-density lesions on pre-operative CT.