Twenty-nine patients with chronic bilateral subdural haematomas were surgically treated during 1966 to 1977. Twenty-four of them (83%) had a history of head injury, which caused unconsciousness in eight cases. The mean interval from trauma to operation was eleven weeks. The mean age of the patients was 60 years. The prevalence of the most commonly encountered symptoms and signs was: headache 72%, mental symptoms 48%, papilloedema 41%, vertigo 31%, nausea 28%, reduced consciousness 28%, walking difficulties 24%, hemiparesis 24%, and paraparesis 14%. The aggregate thickness of haematomas was 34 mm, 36 mm, and 40 mm in age groups of 20--39, 40--59, and over 60 years, respectively. All patients were operated on, four of them only unilaterally. Three patients in the whole series died. Two of them had been operated upon only on one side in the first session, the haematoma of the other side being evacuated 8 1/2 hours and four days later, respectively. Unilateral operation is likely to cause severe distortion of the midline structures and the brain stem and thus aggravates the cerebral situation. Therefore the necessity of simultaneous evacuation of the haematomas on both sides is stressed. The reason for the death of the third patient was delay in diagnosis. All three patients who died belonged to the group of eight patients with a reduced level of consciousness before surgery. Twenty-three of the survivors were fully independent in their daily lives, and three needed some help after operative treatment.