Obesity appears to be associated with an increased risk of breast cancer (BC) and an inferior oncological outcome at the time of diagnosis, with poor outcomes most prominent in cases of triple-negative BC (TNBC). The present study analyzed serum tumor necrosis factor (TNF)-like weak inducer of apoptosis (TWEAK) and TNF receptor associated factor-6 (TRAF) protein expression levels in 48 patients with non-metastatic BC and 26 obese control patients (without BC). The mean age of the cohort was 52.5 years (range, 35-78 years) and the patients had a median body mass index of 33.5 kg/m(2) (range, 30-47 kg/m(2)). In the study population, 27.1% of BC patients were triple negative and 70.8% were hormone receptor (HR)-positive. Median serum TRAF6 expression was 0.90 ng/ml (range, 0.55-1.53 ng/ml) in the 13 TNBC patients and 0.63 ng/ml (range, 0.49-1.22 ng/ml) in the 35 HR-positive BC patients; thus, TRAF6 expression was significantly higher in the TNBC patients compared with the obese control group (0.90 vs. 0.73 ng/ml; P=0.033). Furthermore, median serum TRAF6 expression levels were significantly higher in HR-negative patients compared with HR-positive patients (0.83 vs. 0.62 ng/ml; P=0.002). The present study demonstrated that serum TRAF6 expression levels were increased in TNBC and HR-negative patients with non-metastatic BC compared with HR- and human epidermal growth factor receptor 2-positive cases or the obese healthy control group. Therefore, elevated TRAF6 expression may be a poor prognostic factor in non-metastatic BC. In addition, we propose that progesterone (PR) negativity may be a more useful poor prognosis factor than estrogen receptor (ER) negativity, as TRAF6 expression levels were higher in the PR-negative patients compared with the ER-negative patients.