Development of a testicular adrenal rest tumor (TART) is common in males with congenital adrenal hyperplasia, and it can be an important cause of infertility. In the present study, we observed the prevalence of TARTs, and analyzed its associated factors in patients with 21-hydroxylase deficiency. Testicular ultrasonography was performed in 48 postpubertal male patients aged 10.6 to 27.1 years. To determine whether patients were undertreated, we analyzed the serum 17-hydroxyprogesterone (17-OHP) levels to the time of ultrasonographic measurement and calculated the percentage of measurements when serum 17-OHP level was >10 ng/mL relative to the total number of measurements during the follow-up period. We divided the 6-year period before ultrasonographic measurement (time 0) into three 2-year intervals and calculated the average concentration of serum 17-OHP in each interval to give a -2^nd to 0 year-average concentration (-2-0YAC), -4-2YAC and -6-4YAC. A TART was detected by ultrasonography in 31 of 48 patients (64.6%) and the median maximal cross-sectional area of the TARTs was 0.71 (0.03, 4.95) cm². The corrected final adult height was lower, and -4-2YAC and body mass index were higher in patients with TART than in those without. After controlling for the type of 21-hydroxylase deficiency, hydrocortisone-equivalent dose, age, and -6-4YAC, the size of TART was associated with a high undertreatment percentage with a marginal statistical significance. These results suggest that strict disease control is mandatory and regular examination with testicular ultrasonography is recommended in male patients, regardless of the type of 21-hydroxylase deficiency.