Purpose Breast cancer is one of the most important health problems faced by women. No study was found in the world literature about the eating behavior of women with breast cancer. Therefore, the aim of this study was to determine whether breast cancer patients and healthy controls differ in their orthorexia nervosa levels and to determine any factors that affect orthorexia nervosa (socio-demographic variables and nutritional habits). Method The data were collected using a face-to-face interview technique between May 2018 and March 2019 at outpatient clinics and a family health center in Turkey. The data of the study were collected using personal information form and the Orthorexia Nervosa Scale (ORTO-15). A linear regression analysis was performed to investigate the effects of socio-demographic variables and nutritional habits of women on the risk of orthorexia nervosa. Results Breast cancer patients had significantly lower ORTO-15 scores (i.e., a higher orthorexia risk) than the healthy controls. For the cancer patients, a regression analysis revealed that ORTO-15 scores were significantly associated with education level, organic food consumption status, receipt of social support for care, and presence of a chronic disease other than cancer. In the healthy controls, body mass index and education level were the primary predictors of ORTO-15 scores. Conclusion The higher orthorexia risk of cancer patients has implications for these patients that could be improved through nutritional counseling.