The diagnostic utility of F-18-fluorodeoxyglucose (FDG)-positron emission tomography (PET) for breast cancer is controversial. The histological type or tumor size of breast cancer has been reported to be associated with a greater likelihood of positive FDG uptake. Compared to invasive ductal carcinomas (IDCs), invasive lobular carcinomas (ILCs) have a lower level of FDG uptake and are detected at a significantly lower sensitivity. The role of preoperative FDG-PET for ILCs may, thus, be limited. Few data evaluating the significance of FDG-PET in ILCs are available. Here, we evaluated the clinical significance of FDG-PET for ILC patients. We retrospectively investigated the cases of 196 consecutive patients with primary breast cancer who were diagnosed as having ILC (n=15) or IDC (n=181) and underwent FDG-PET preoperatively. Fifteen (7.7%) of patients were histopathologically diagnosed as ILC. A univariate analysis revealed that tumor size, extent of tumor, estrogen receptor (ER) expression and progesterone receptor (PgR) expression were significantly different between the ILC and IDC groups. The maximum standardized uptake value (SUVmax) values of the primary tumors were not significantly different between the two groups but, regardless of the larger size of tumor or ductal spread, the SUVmax was relatively lower in the ILC group compared to the IDC group. The tumors in two ILC cases showed no FDG uptake. Among the ILC cases, there were linear associations between SUVmax and tumor size and between SUVmax and the nuclear grade by Pearson correlation (r=0.447, p=0.048 and r=0.519, p=0.024, respectively). Our findings imply that the preoperative FDG uptake in ILC may be reflective of the tumor size and the nuclear grade of the tumor. FDG uptake may be useful and predictive of aggressive features or prognosis in ILC patients.