Fresh evidence adds weight to suggestions that loneliness makes cancer both more likely and deadly.
Work in Proceedings of the National Academy of Science shows social isolation tips the odds in favour of aggressive cancer growth.
Rodents kept alone developed more tumours - and tumours of a more deadly type - than rats living as a group.
The researchers put it down to stress and say the same may well be true in humans.
Cancer experts say more work is needed to prove such a link in people.
Lead investigator Gretchen Hermes, of Yale University, said: "There is growing interest in relationships between the environment, emotion and disease.
"This study offers insight into how the social world gets under the skin."
Doctors already know that cancer patients who are depressed tend to fare worse in terms of survival.
And previous research has suggested that social support can improve health outcomes for patients with breast cancer.
In the latest study, the researchers found that isolation and stress trebled the risk of breast cancer in the naturally sociable Norway rats.
Outcast rodents developed 84 times the amount of tumours as those living in tight-knit social groups, and the tumours also proved to be more aggressive.
The isolated mammals also had higher levels of the stress hormone corticosterone and took longer to recover from a stressful situation than fellow Norway rats.
The researchers ultimately hope their work will help cancer patients.
Co-researcher Martha McClintock, a psychologist at the University of Chicago, said: "We need to use these findings to identify potential targets for intervention to reduce cancer."
Ed Yong, of Cancer Research UK, said: "This study was done in rats.
"Overall, research in humans does not suggest there is a direct link between stress and breast cancer.
"But it's possible that stressful situations could indirectly affect the risk of cancer by making people more likely to take up unhealthy behaviours that increase their risk, such as overeating, heavy drinking, or smoking."