Mental health disorders ranging from depression to epilepsy, will be the second most common cause of death and disability by 2020, say experts.
The World Health Organzation estimates the numbers affected by mental and neurological disorders will surge over the next 20 years.
They currently affect 400 million people across the world.
The WHO currently ranks depression in fifth place.
The United Nations health agency said the rise was probably due to people living more stressful lives, and suffering more poverty and violence.
Alzheimer's disease, which predominantly affects the elderly, is expected to increase as people live longer.
The World Health Organization was launching its campaign for this year called: "Stop exclusion - Dare to care", which aims to remove the stigma from mental illness.
Dr Derek Yach, from the WHO said: "This (campaign) is overdue, given that mental health is a major cause of disability, family and community distress and loss of production."
Dr. Benedetto Saraceno, director of WHO's department of
mental health and substance dependence, said: "There is a common myth that mental health problems are those of rich, industrialised countries, a luxury.
"But mental and neurological problems are equally important in rich and poor countries."
Ischemic heart disease, now the sixth leading cause of death, is predicted to cause the largest number of deaths.
Acute lower respiratory infections currently cause the largest number of deaths around the world, but the WHO predicts infectious disease numbers are expected to fall.
Depression, which is often genetic, hits roughly twice as many women as men.
The WHO estimates United States and Japan have the highest numbers with depression.
The lowest rates are in Africa.
Dr Yach said: "Surveys in Brazil, Zimbabwe, India and Chile show the more impoverishment - measured by hunger, level of debt and education - the higher the prevalence of common mental disorders."
There are around one million suicides worldwide every year, out of 10m attempts.
The top ten countries for numbers of suicides happen to be former Soviet Union countries.
Mental and neurological disorders represent 11% of the "global burden of disease", which is expected to rise to 14% in 2020.
Dr Saraceno said 70% of those suffering from major depression can fully recover if properly treated.
"We can expect a decrease in suicides if we are properly treating depressed people."
Schizophrenia, another mental illness, affects 45 million people worldwide.