Several case-control studies have reported head injury to be more common among patients with Alzheimer's disease (AD) than healthy elderly controls. The present study sought to determine whether milder head injury is also a risk factor for AD. Furthermore, it was hypothesized that head injury would be more common among AD patients without a genetic risk for the disease. History of head injury in 68 consecutive cases of probable or definite AD and 34 non-demented control subjects was ascertained from their spouses. Head injury was reported in 20 of the AD patients (29%), and in only one control subject (2.9%) (odds ratio = 13.75). Twenty per cent of the familial and 43.5% of the sporadic AD cases reportedly had a premorbid head injury (odds ratio = 3.08). Head injury had no effect on age of dementia onset. The results indicate that head trauma may be a predisposing factor to AD, particularly in the absence of a clear genetic contribution.