Abstract Background Twelve percent of the Dutch population consists of non-Western immigrant women with an incidence of breast cancer that has risen from 2,9% in 2005 to 5% in 2015. For the second generation of these women the incidence of breast cancer is expected to meet the native Dutch population rates soon, partly due to adoption of Western style dietary habits and physical activity. Chemotherapeutic treatment (CT) for breast cancer is associated with increased body fatness and interventions to prevent this increase are currently explored. Whether perceptions on cancer and treatment differ between non-Western immigrant women than for Dutch women needs to be evaluated before these interventions can be set up. This study aimed to explore cultural and religious differences on women's perceptions of the diagnosis of breast cancer and changes in physical activity and eating habits during chemotherapy treatment. Methods A longitudinal qualitative multiple case study was conducted. Newly diagnosed women with breast cancer were recruited and purposively selected (n=23, non-immigrant) from six hospitals in the Netherlands. Semi structured interviews were conducted three times (in total 69): before start of CT, halfway and after CT. In addition 38 women (20 non-immigrant and 18 immigrant) were recruited and interviewed after finishing CT. All interviews (n=107, from 61 women) were audiotaped and transcribed verbatim. A thematic content analysis approach was used. Results All 43 non-immigrant women, mean age 51,1 yrs. and 18 immigrant women mean age 43,2 yrs., experienced known side effects from CT. Loss of hair and sometimes the breast appeared to be especially for immigrant women a shocking experience and for some even a loss of femininity. Most of the women perceived to have received incomplete and often unclear information from hospitals about weight and CT treatment. Immigrant women participating the Ramadan during treatment encountered resistance from their physicians. Weight gain during period of CT was higher among immigrant women (mean 13,1 kg, 4-28 kg) than among non-immigrant women (mean 2,5 kg, 2-9 kg). Although both groups said to be less physically active and complained about fatigue, non-immigrant women trying to maintain daily structure and were more active with their diet than most immigrants. Immigrant women expressed cancer as a taboo in their culture impeding them to talk openly about their illness, only when side effects of CT such as hair loss were visible they found it inevitable. Cancer was associated with death at time of diagnosis. For most immigrants and a few non-immigrants this was considered a religious ordeal from Allah or God, these women perceived less influence on their behavior during treatment. Most non-religious women perceived breast cancer as bad luck, stress or heredity. Conclusion Especially immigrant women experienced little respect for their culture and insufficient information about treatment. Non-immigrants had a need to actively contribute to their treatment while this need was less obvious for immigrants. Probably because they felt not encouraged by their religion and culture. Citation Format: de Kruif AJ, Derks MJ, de Boer MR, Winkels R, Visser M, Kampman E, Westerman MJ. Cultural and religious differences during breast cancer treatment between Dutch and non-Western immigrant women. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P1-10-28.