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Studies of obesity and survival among patients with breast cancer produce conflicting results, possibly because of heterogeneity by molecular subtype.This study examined whether the association of body mass index (BMI) at diagnosis with breast cancer recurrence and survival varied across subtypes defined by PAM50 (Prediction Analysis of Microarray 50) gene expression. Included were 1559 Kaiser Permanente Northern California members ages 18 to 79 years who had PAM50 assays and were diagnosed with American Joint Committee on Cancer stage I through III breast cancer from 1996 to 2013. Patients reported weight and height. Cox regression models were adjusted for age, menopause, race/ethnicity, stage, and chemotherapy.Over a median of 9 years (maximum, 19 years), 378 women developed recurrent disease, and 312 died from breast cancer. Overall, BMI was not associated with breast cancer recurrence or survival when controlling for subtype (eg, the hazard ratio per 5 kg/m2 of BMI was 1.05 [95% confidence interval, 0.95-1.15] for breast cancer-specific death). However, associations varied by subtype. Among women with luminal A cancers, those who had class II/III obesity, but not class I obesity or overweight, had worse outcomes. When women who had a BMI ≥35 kg/m2 were compared with those who had a BMI from 18.5 to <25 kg/m2, the hazard ratio was 2.24 (95% confidence interval,1.22-4.11) for breast cancer-specific death and 1.24 (95% confidence interval, 1.00-1.54) for recurrence. There was no association within luminal B, basal-like or human epidermal growth factor over-expressing subtypes.Among patients who had accurately classified breast cancer subtypes based on gene expression, a BMI ≥35 kg/m2 was adversely associated with outcomes only among those who had luminal A cancers. Research is needed into whether tailoring recommendations for weight management to tumor characteristics will improve outcomes.In the largest prospective study to examine the association of obesity and breast cancer outcomes separately by PAM50 subtype, extreme obesity doubles the risk of breast cancer death among women with luminal A tumors, and there is no association of body mass index and breast cancer outcomes for other subtypes. Future research should investigate the potential of a precision oncology approach that targets lifestyle intervention according to individual characteristics of the patient and disease.