Objectives. To search for any relation between plasma adiponectin levels and the cellular differentiation or progression of prostate cancer (PCa). PCa is becoming an increasingly important public health problem, particularly for those countries with a trend toward an aging population. Because insulin resistance in the setting of obesity is associated with the development of PCa, we hypothesized that decreased adiponectin levels might underlie the association between PCa and obesity/insulin resistance. Methods. In this study, we investigated plasma adiponectin levels in 30 patients with PCa, 41 subjects with benign prostatic obstruction, and 36 healthy individuals. The body mass index and age of the groups were similar. Patients with PCa were stratified into two groups according to the spread of the disease as organ-confined and advanced disease and into three groups according to grade (low, intermediate, and high grade determined by a Gleason sum of less than 5, between 5 and 7, and more than 7, respectively). Results. Plasma adiponectin levels were significantly lower in the PCa group than in the benign prostatic obstruction group or controls (P <0.001 for both). Additionally, the plasma adiponectin levels were significantly lower in the advanced disease group than in the organ-confined PCa group (P = 0.012). Significant negative associations were found between plasma adiponectin levels and prostate-specific antigen levels or biopsy Gleason scores in the PCa group. The plasma adiponectin levels of those with high-grade PCa were also significantly lower than those for both the low-grade and intermediate-grade groups (P <0.001 for both). Conclusions. The results of the present study imply that plasma adiponectin levels are not only lower in patients with PCa but are also negatively associated with the histologic grade and disease stage. Future prospective studies are recommended to establish any causal relation between PCa and plasma adiponectin levels. © 2005 Elsevier Inc.