To evaluate the operative mortality and morbidity of definitive intracranial microsurgical aneurysm obliteration as a function of timing of early operative intervention and as a function of clinical condition at the acute state, we retrospectively review 164 consecutive patients who underwent surgery within 72 hours following haemorrhage. The series was divided into four operation periods (0-6, 6-12, 12-24, 24-72 hours), and patients were graded according to five clinical conditions described by Hunt and Hess. The mortality of the individual clinical condition at each operation period was to great extent independent of the timing of operation, and there was a distinct correlation between the surgical results and the form of bleeding visualized by C.T. In poor condition (grade 3, 4, and 5) patients, satisfactory surgical results were obtained in patients in whom cisternal blood clots, intracerebral haematoma, and subdural haematoma had been shown by C.T. The optimum operation times for each group were suggested.