Background/Purpose: Ureteropelvic junction (UPJ) obstruction is the most common cause of congenital hydronephrosis. Previous studies have reported that the excess amount of collagen restricting mobility and resiliency of the UPJ is the result of an impaired collagen production by anomalous smooth muscle cells (SMCs). Our purpose was to evaluate the role of SMC differentiation in the pathogenesis of UPJ obstruction. Methods: Surgical specimens of UPJ from 21 patients (8 girls/13 boys) who were subjected to dismembered pyeloplasty were examined immunohistochemically using monoclonal antibodies against smooth muscle (SM) myosin heavy chain isoforms including SM1, SM2, and SMemb. The age ranged from 1 month to 13 years. Ureteropelvic walls taken from 14 forensic autopsy cases, with no urological abnormalities, served as agematched control group. Results: The immunohistochemical expression of SM1 and SM2 in UPJ obstruction was signifi- cantly increased when compared with controls (P < .05). In contrast, there was no statistical difference of expression of SMemb. Conclusion: Our findings supported the hypothesis that the primary anomaly in UPJ obstruction may be attributed to a malfunction of SMCs in the ureter. Background / Purpose: Ureteropelvic junction (UPJ) obstruction is the most common cause of congenital hydronephrosis. Previous studies have reported that the excess amount of collagen restricting mobility and resiliency of the UPJ is the result of an impaired collagen production by anomalous smooth muscle cells Methods: Surgical specimens of UPJ from 21 patients (8 girls / 13 boys) who were subjected to dismembered pyeloplasty were examined immunohistochemically using monoclonal antibodies against smooth The age ranged from 1 month to 13 years. Ureteropelvic walls taken from 14 forensic autopsy cases, with no urological abnormalities, served as agematched control group. Results: The immunohistochemical activity of muscle (SM) myosin heavy chain isoforms including SM1, SM2, and SMemb. expression of SM1 and SM2 in UPJ obstruction was signifi- cantly increased when compared with controls (P <.05). In contrast, there was no statistical difference of expression of SMemb. Conclusion: Our findings supported the hypothesis that the primary anomaly in UPJ obstruction may be attributed to a malfunction of SMCs in the ureter.