Piling on the kilos over several decades increases the risk of obesity-related cancers, research has shown.
British scientists looked at the link between obesity and cancer in around 300,000 Americans - who were monitored between the ages of 18 and 65.
During that time, some people gained little weight while others became "morbidly" obese - meaning they were so fat it endangered their health.
The population was then followed for an average of 15 more years while cancer rates were recorded.
Men whose Body Mass Index (BMI) rose from around 22 to 27 had a 50 per cent increased risk of developing obesity-related cancer compared with men who stayed within a healthy weight range.
Women who went from a BMI of 23 to around 32 experienced a 17 per cent increase in risk. BMI is a measurement that relates height and weight.
Being overweight or obese is linked to a wide range of cancers - including bowel, breast and pancreatic cancer.
Several obesity-related cancers, such as womb and ovarian cancer, only affect women.
Lead scientist Dr Hannah Lennon, from the University of Manchester, said: "This research shows how important it is to look at weight gain over a person's lifetime - to give a clearer picture of cancer risk through life compared to assessing someone's BMI at a single point."
The findings were presented at the National Cancer Research Institute's (NCRI) Cancer Conference in Liverpool.
