Serum C-reactive protein (CRP) concentrations have been reported to be associated with body fat, especially visceral fat accumulation, but most studies up to now have been conducted on non-diabetic subjects. In this study, we investigated the association between the serum CRP concentrations and parameters of adiposity and insulin resistance in both Japanese type 2 diabetes patients and non-diabetic subjects. A total of 248 Japanese subjects (140 type 2 diabetes patients and 108 non-diabetic subjects) were enrolled for the study. The degree of insulin resistance was estimated by the homeostasis model assessment (HOMA-R) method. Fat accumulation was evaluated by measuring visceral and subcutaneous fat areas at the level of the umbilicus in abdominal CT scans. To assess hepatic fat content, the ratio of CT attenuation value of the liver to that of the spleen (L/S ratio) was calculated. Serum CRP was found to be significantly correlated with various indices of adiposity, including L/S ratio, visceral fat area (VFA), subcutaneous fat area (SFA), and HOMA-R, in both the diabetic patients and the non-diabetic subjects. After adjustment for five variables (age, gender, serum CRP, HbA1c, and smoking), serum CRP was still significantly correlated with L/S ratio, VFA, SFA, and HOMA-R in the diabetic patients. We also found that changes in serum CRP concentrations were correlated with changes in the VFA and SFA at 1 year after the baseline in 24 diabetic patients. We conclude that serum CRP may be closely related to the degree of liver steatosis and visceral fat accumulation in Japanese type 2 diabetes mellitus patients.