Objectives: Obese patients with prostate cancer have more aggressive tumors and, in some studies, more prostate cancer-specific deaths. This study was designed to assess the relationship between body mass index (BMI) and treatment patterns of prostate cancer patients. Methods: We identified 5041 men with clinically localized prostate cancer (T1-3a, N0M0) who received their first treatment between 1995 and 2006. We derived the odds ratios (OR) for the likelihood of receiving each type of therapy compared with radical prostatectomy by BMI categories using multinomial logistic regression. In our analysis we controlled for age at diagnosis, race/ethnicity, education level, clinical risk category, and number of co-morbidities. Results: A total of 28.1% of patients were classified as normal BMI, 50.5% were overweight, 16.5% were obese, and 4.8% were very obese. The adjusted OR of receiving nonsurgical therapies (brachytherapy, external radiation, primary androgen deprivation, and active surveillance) increased relative to radical prostatectomy for increasing obesity (P = 0.003). Compared with the patients with normal BMI, very obese patients were more likely to receive brachytherapy (OR 1.59, 95% confidence interval [CI] 1.01 to 2.52), external radiation (OR 1.29, 95% CI 0.73 to 2.26), primary androgen therapy only (OR 1.77, 95% CI 1.12 to 2.81), and active surveillance (OR 1.06, 95% CI 0.52 to 2.17) compared with radical prostatectomy. Conclusions: In a large cohort of American prostate cancer patients, a significant trend toward nonsurgical treatment modalities was apparent with increasing BMI. © 2008 Elsevier Inc. All rights reserved.