Objectives: To assess the clinicopathologic features of Multicentric Castleman disease (MCD) patients in Japan. Methods: We assessed baseline data for 342 Japanese MCD patients with a biopsy-proven diagnosis, enrolled in a prospective, observational study for tocilizumab treatment. Results: Of 342 patients, 86.0% had plasma-cell type. None had a family history of MCD. Median disease duration of MCD was 3.7 years. Mean body weight and body mass index tended to be lower than those in the general Japanese population. The most common clinical presentations besides lymphadenopathy included fatigue (61.7%), pulmonary involvement (42.7%), and splenomegaly (41.8%). Secondary amyloidosis was reported in 34 patients (9.9%). Laboratory abnormalities included decreased hemoglobin and albumin, and increased acute-phase proteins, serum immunoglobulins, and interleukin-6 (IL-6). IL-6 levels among the MCD patients tested in this study were correlated with levels of albumin, hemoglobin, triglyceride, total cholesterol, C-reactive protein, fibrinogen and immunoglobulin G (Spearman’s correlation coefficient, |r| = 0.28–0.59). Conclusion: The clinical features and laboratory abnormalities are similar to those previously reported in other countries, besides higher rates of pulmonary involvement, secondary amyloidosis, and ECG abnormalities. Our results imply that IL-6 is involved in MCD pathogenesis. These findings would be informative for diagnosis and appropriate treatment for MCD.