A biological warning sign of a life blighted by major depression has been identified for the first time in teenage boys.
Boys with a combination of depressive symptoms and raised levels of the stress hormone cortisol were up to 14 times more likely to be affected than those with neither trait, scientists found.
The diagnostic marker could help individuals most at risk of serious depression to get earlier treatment, the researchers believe.
"Depression is a terrible illness that will affect as many as 10 million people in the UK at some point in their lives," said study leader Professor Ian Goodyer, from Cambridge University.
"Through our research, we now have a very real way of identifying those teenage boys most likely to develop clinical depression. This will help us strategically target preventions and interventions at these individuals and hopefully help reduce their risk of serious episodes of depression and their consequences in adult life."
The scientists measured cortisol levels in the saliva of almost 2,000 young people aged 12 to 19 and studied self-reported data about experiences of depression.
Rates of clinical depression and other psychiatric disorders in the group were recorded between 12 months and three years later.
Boys with raised levels of cortisol in the morning and pronounced symptoms of depression were 14 times more likely to develop serious depressive illness than those with normal cortisol and few symptoms.
The trend was much weaker for girls, suggesting gender differences in the way depression develops.
Dr John Williams, head of neuroscience and mental health at the Wellcome Trust, which funded the study, said: "Progress in identifying biological markers for depression has been frustratingly slow, but now we finally have a biomarker for clinical depression. The approach taken by Professor Goodyer's team may yet yield further biomarkers. It also gives tantalising clues about the gender differences in the causes and onset of depression."
The findings appear in the journal Proceedings of the National Academies of Sciences.
Serious clinical depression is distinct from having occasional depressive symptoms or "feeling blue" and is regarded as a genuine illness.
Clinical depression is defined by a range of specific symptoms, including fatigue, feelings of worthlessness, loss of interest, and sleep disturbances.