Patients who have suffered a head injury may be more likely to develop depression, say researchers.
Mental problems are common after head injury
A small study published in a US journal revealed that head injury patients had a higher risk of problems than patients who had suffered other injuries.
A third of those examined by doctors in Iowa City showed signs of depressive illness within a year.
Patients who suffered major bouts of depression had differences in the physical structure of their brains.
The mechanism by which a head injury might cause changes of mood and behaviour are poorly understood.
However, many people who have suffered a head injury become more irritable and aggressive, or endure anxiety attacks or full-blown clinical depression.
The Iowa study looked at 91 patients who had experienced a "traumatic brain injury", and 27 who had suffered other injuries - but not a head injury.
They were checked every few months to see if they were suffering any kind of mental problems.
Of the head injury patients, 33% were diagnosed with "major depressive disorder" during the first year.
Many of these also had symptoms of anxiety and 56% showed aggressive behaviour.
Depression was significantly more likely among the head injury patients compared with those with other injuries.
Looking at the records of more than 900 other patients with brain injury suggested that half of those with severe or moderate injuries suffered some kind of psychiatric illness in the first year.
Among hundreds of patients with other injuries, only one in five were recorded as having psychiatric problems.
Dr Huw Williams, from Exeter University, told BBC News Online that doctors were becoming increasingly aware of the mental problems faced by head injury patients even after the physical side of their injury had disappeared.
He said: "This backs up what other studies have been saying about depression and head injuries. There is a very high rate.
"It reinforces the importance of good neuro-rehab after this kind of injury. Both the NHS and organisations such as Headway have a big role to play in helping people in this situation.
"At the moment, neuro-rehab services are best described as patchy."
The research was published in the Archives of General Psychiatry.