Purpose We investigated whether PET indices measured by $^{18}F$-fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT) can predict prognosis in patients with operable primary breast cancer. Methods We reviewed 53 patients with operable primary breast cancer who underwent pretreatment FDG PET/CT. PET indices, maximum standardized uptake value (SUV) and metabolic tumor volume (MTV), were measured in the primary breast tumor (P), metastatic lymph nodes (N) and total tumor (T). The Cox proportional hazards model was used with age, tumor size, clinical lymph node status, method of surgery, presence or absence of neoadjuvant chemotherapy, histological type, histological grade, hormone receptors and HER2 status to predict disease-free survival (DFS) and overall survival (OS). Results Median follow-up period was 50 months (range, 17-73 months), during which 17 patients had recurrent disease and nine of whom died. The univariate analysis showed that high SUV of N ($N_{SUV}$, P=0.011), MTV of N ($N_{MTV}$, P=0.011) and MTV of T ($T_{MTV}$, P=0.045) as well as high histological grade (P=0.008), negative estrogen (P=0.045) and negative progesterone (P=0.029) receptor status were associated with shorter DFS. High $N_{SUV}$ (P=0.035), $N_{MTV}$ (P=0.035) and $T_{MTV}$ (P=0.035) as well as high histological grade (P=0.012) and negative estrogen receptor status (P=0.009)were associated with shorter OS. $N_{SUV}$, $N_{MTV}$ and $T_{MTV}$ were found to be significantly associated with high histological grade (P=0.005). However, those failed to be statistically significant prognostic factors on multivariate analysis. Conclusions PET indices seem to be useful in the preoperative evaluation of prognosis in patients with operable primary breast cancer. $N_{SUV}$, $N_{MTV}$ and $T_{MTV}$ might be considerable factors associated with patient outcome in operable breast cancer.