Over 80% of people with Parkinson's disease frequently experience depression, a European survey finds.
Parkinson's disease is a degenerative, neurological condition
But the poll of 500 patients with mild-to-moderate forms of the disease found 40% rarely - or never - talked to their doctors about depression.
And two thirds of doctors polled said they considered other symptoms were more important than depression.
But Parkinson's experts said depressive symptoms were as important as motor problems for people with the disease.
Around one in 500 people in the UK have Parkinson's disease.
Around 10,000 new cases are diagnosed annually, with one in 20 affected someone under 40.
The most well-known symptom is tremors in the arms and legs.
But depression can stem from people's feelings about their condition, or as another symptom caused by the neurological effects of the disease.
This survey covered patients in France, Germany, Italy, Spain and the UK.
Normal outlook 'difficult'
Researchers also spoke to 500 specialist doctors in the same countries.
Virtually all said the majority of their patients "often" or "sometimes" experienced symptoms of depression.
But 49% said such symptoms were difficult to recognise.
Doctors said the main reason they did not discuss depression was that they felt that patients did not rate these symptoms as being as important as other aspects of their condition.
But patients said depression was almost as significant for them as movement problems.
Mary Baker, president of the European Parkinson's Disease Association, said: "This survey has confirmed what we've been hearing from people with PD, and the people who care for them, for some time.
"In many cases, it's not the symptoms that one normally associates with Parkinson's disease that cause the most distress.
"When your mood is affected, it can be very difficult to maintain a normal outlook on life."
She added: "Those who are caring for people with PD often report that seeing their loved one feeling depressed is one the hardest aspects of the condition to deal with."