In a study of more than 70,000 women, optimists were less likely to get fatal cancer, heart disease, lung conditions and stroke in their retirement years.
Although some of the association is explained by healthier life choices and behaviours, experts believe a positive mental attitude is powerful in itself.
And even if you lack a natural sunny disposition, optimism can be learned.
Encouraging people to imagine a bright future could be a good medicine to boost public health, say the authors of the study in the American Journal of Epidemiology.
The researchers looked at health data collected from a large US study of working and retired female nurses.
The participants had been asked to rate how optimistic they perceived themselves to be on a scale of zero to 24.
The healthy women, who ranged in age from 58 to 83, were monitored over the next eight years and any disease-related deaths were recorded. There were 4,566 deaths overall.
Higher optimism was linked with lower death risk, even after controlling for other factors such as whether the woman was married, came from a richer or poorer family background, or had a history of diabetes, high blood pressure, high cholesterol or depression.
A significantly lowered risk was seen for deaths from a number of causes, including heart disease, stroke, cancer, respiratory disease and infection over the course of the study period.
The researchers believe optimism may have a biological effect on the body as well as a psychological one, although they didn't look at this.
Other studies have linked a positive outlook to lower inflammation, for example, and better heart health.
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Investigator Dr Eric Kim, from the Harvard TH Chan School of Public Health, said there were several strategies people might want to try for an optimism boost:
Or you could try a course of mindfulness or some sessions of talking therapy.
He said: "Twin studies suggest up to 25% of optimism might be genetic or inherited which would mean up to 75% could be modifiable."
Women who look on the bright side of life cut their risk of many deadly diseases, according to researchers.