Abstract Background: Breast cancer in younger women (≤ 40) is associated with greater physical and psychological morbidity than in older women. No study to our knowledge has examined the effect of a breast cancer diagnosis in young women on their relationship with their mothers, or the support needs of these mothers. Methods: We developed and pre-tested a self-administered questionnaire on 10 survivors of breast cancer diagnosed ≤ age 40 for clarity, content and sensitivity (San Antonio, 2011). Then, consecutive recurrence-free women age ≤ 40 at diagnosis, within 4 years of a breast cancer diagnosis, whose mothers were alive at diagnosis, were asked to complete the modified questionnaire in a medical oncology follow-up clinic. Results: Of 110 eligible daughters approached from 07/2011–05/2012, 90 with a mean age 36 (range 21–43) participated in the study. Stage at diagnosis: 1–23 (26%), 2–46 (51%) 3–21 (23%). Ethnicity: Asian: 29 (32%), Caucasian 43 (48%), Black 2 (2%), Mix/Other 16 (18%). At diagnosis, 15 were living with their mothers, 44 were not living in the same city including 23 who were in different countries, 7 of whom were not informed of the diagnosis as the daughters did not want to worry them. Mean age of the 88 mothers alive at time of study was 64 (range 48–84) and 16 had previously had breast cancer. Illness attributions: Eight blamed their mothers for their developing breast cancer, and 22 believed their mothers felt responsible to some extent (overlap in 4). Supports: Of the 43 daughters who had turned to their mothers for emotional support over the year prior to diagnosis, all but one did so after diagnosis, as did 20% of those who had not turned to mothers over the year prior to diagnosis. In 11 cases, the daughters turned to their mothers before approaching anyone else for support. Of the 83 daughters who informed mothers of their diagnosis, 76 (92%) reported their mothers were emotionally and/or practically supportive. The 4 most difficult issues faced by daughters were fatigue, anxiety, breast loss and menopausal symptoms, with 70 daughters discussing at least some issues with their mothers. Of the 35 working mothers, 27 took time off to support their daughters. Nineteen mothers slept over or moved in with the daughters during their treatment, 8 of whom had been living in a different country. Forty-four (44/83=53%) daughters reported that the breast cancer diagnosis had a favorable impact on their relationship with their mothers. Formal supports for mothers: Thirty-two (32/83=39%) reported their mothers did not have adequate psychosocial support, and 59 (59/90=66%) indicated health care professionals could help mothers by providing brochures on caring for a daughter with breast cancer, having professionally led education sessions, as well as support groups. Conclusion: Mothers are an important source of support for young breast cancer daughters, and most daughters perceived that the diagnosis resulted in the mother-daughter relationship becoming closer. However, the physical and emotional toll on mothers appears to be high. Future studies should address the effects of a breast cancer diagnosis in a young daughter from the mothers' perspective, and the benefit of formal, culturally sensitive supports for these mothers. Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P6-08-04.