The objective of the study was to evaluate whether or not sexual development differs between boys with prepubertal obesity and boys of normal weight. We enrolled healthy obese and normoweight schoolboys from Shenyang City, Liaoning, China. Eligible boys were at Tanner stage 1 and 8 years of age at baseline. We measured testosterone and estradiol concentrations in the saliva and assessed auxology annually for 4 years. In all 56 obese and 56 normoweight boys, the height, weight, body composition, and sex organ volume increased with age. The percentages of body fat, fat mass, and lean mass were all higher in obese than in normoweight boys. The mean testicular volume was smaller in obese boys than in normoweight boys. The sex hormone concentrations increased with age, except for testosterone in obese boys in year 3 of follow-up. In year 4, estradiol concentrations were significantly higher in obese boys than in normoweight boys. Excessive adiposity in prepubertal boys might affect testicular volume, possibly because of high estradiol and low testosterone concentrations.