Post-menopausal women who have Type 2 diabetes appear to have a 27% greater risk of developing breast cancer, experts say.
Being obese or overweight is linked to both conditions.
But cancer experts say there may be a direct connection between the two.
These studies involved more than 56,000 women with breast cancer.
Post-menopausal women with Type 2 diabetes had a 27% increased risk of breast cancer but there was no link for pre-menopausal women or those with Type 1 diabetes.
The authors have also suggested that a high body mass index (BMI), which is often associated with diabetes, may be an underlying contributing factor.
The President of the International Prevention Research Institute, Prof Peter Boyle who led the study, said: "We don't yet know the mechanisms behind why Type 2 diabetes might increase the risk of breast cancer.
"On the one hand, it's thought that being overweight, often associated with Type 2 diabetes, and the effect this has on hormone activity may be partly responsible for the processes that lead to cancer growth.
"But it's also impossible to rule out that some factors related to diabetes may be involved in the process."
It's not clear whether there's a causal link between diabetes and the risk of breast cancer in post-menopausal women.
But as we know that having a high BMI can contribute to an increased risk of both Type 2 diabetes and breast cancer, it makes sense for women to try and maintain a healthy weight.