The percentage of myeloma cells in bone marrow is subsequently an important index of disease in patients with multiple myeloma (MM). Bone marrow myeloma cells can be detected by strong CD38/CD138 positivity and light scatter characteristics using flow cytometry. The aim of the study was to evaluate the relationship between the degree of F-18 fluorodeoxyglucose (F-18 FDG) uptake and the percentage of CD38/CD138 expressing myeloma cells in the bone marrow of patients with MM. A total of 31 patients with MM (14 females and 17 males, mean age 59.5 +/- 1.9 years, range 29-80 years) were included in the study. All patients underwent FDG-positron emission tomography/computed tomography (PET/CT) scan within 2 weeks after the completion of the usual staging workup for MM, consisting of X-ray skeletal survey and hematological/biochemical parameters including complete blood count, liver and kidney function test, erythrocyte sedimentation rate, serum immunoglobulins, urine light chain excretion, C-reactive protein, beta 2-microglobulin, and bone marrow plasma cell infiltration. In all patients, flow cytometry was performed for assessing the percentage of CD38/CD138 expressing myeloma cells in the bone marrow samples. The extent of bone marrow FDG uptake on PET/CT scans was visually graduated using a qualitative scoring system as extension score (E-score) and also a semiquantitative scoring system defined as mean standardized uptake value (mSUV) of both femora. There was a statistically significant positive correlation between the percentage of CD38/CD138 expressing plasma cells in bone marrow and both mean qualitative (r = 0.616) and semiquantitative (r = 0.755) results of F-18 FDG uptakes. mSUV and E-score of bone marrow FDG uptake values were also correlated with serum beta-2-microglobulin levels (r = 0.523 and r = 479, respectively). mSUV of bone marrow FDG uptake values were also negatively correlated with serum albumin levels (r = -0.424), whereas there was no correlation between E-score and albumin levels. In conclusion, our results indicate that increased F-18 FDG uptake of bone marrow is related to the percentage of plasma cell infiltration of bone marrow in patients with MM. Therefore, F-18 FDG-PET/CT study may be a useful tool for predicting the levels of myeloma cells in bone marrow, and an additional analysis of FDG uptake of bone marrow on FDG-PET/CT scans should be performed in patients undergoing PET studies during the initial staging, evaluating the therapy response, and monitoring patients with MM.