What causes mastitis? My daughter in law is a new mum, with a 10 week old baby, who she is breast feeding. She has been diagnosed with mastitis. She has been told, by well meaning older folk that mastitis is caused when demand feeding, and that she should only be feeding 4 hourly. I think she is doing a great job, and don t want to join the group of ill informed advice givers.... Is mastisis linked to failing to empty the breast with a feed? I was unable to breastfeed when I had my turn, so can t draw on any experience from way back then, but many friends did breastfeed, and I was certainly around when various breastfeeding issues were discussed.