Background/Aims: To examine the role of apoptosis in experimental unilateral ureteral obstruction (UUO). Methods: Rat kidneys were examined 3, 7 and 11 days following UUO or sham operation (SO). Tissue was immunohistochemically stained for α-smooth muscle actin (α-SMA), proliferating cell nuclear antigen (PCNA), Bcl-2 and Bax proteins. Apoptotic analysis was carried out in kidney sections using in situ end labelling of endonuclease cleaved DNA. Results: The relative volume (Vv) of cortical interstitium and interstitial α-SMA increased progressively following UUO. ED1-positive monocytes/macrophages peaked at day 7 and significantly decreased at day 11. PCNA-positive cells in tubulointerstitium were significantly increased on day 3. Staining returned to the level of the SO group by day 11, meanwhile those in the interstitium remained much higher than baseline. TUNEL-positive cells were persistently raised following UUO. Transient tubular cell proliferation seemed unable to counteract the apoptosis since tubular atrophy was apparently present by day 11 of UUO. However, interstitial cell proliferation was high enough to overwhelm apoptosis, particularly with respect ot myofibroblasts, since α-SMA immunostaining and Vv remained elevated. The ratio of the number of PCNA-positive cells to apoptotic cells formed a predictive pattern for the staining score of interstitial α-SMA (R<sup>2</sup> = 47.23%, p < 0.05) and Vv (R<sub>2</sub> = 49.93%, p < 0.05). Tubular Bcl-2 immunostaining peaked on day 3, and then gradually decreased to baseline by day 11. The expression of Bax protein was inhibited on day 3 when compared with that of the SO group, but increased with time following UUO. Conclusion: These findings suggest an important role for apoptosis and its regulatory proteins in the processes of tubular atrophy and fibrogenesis following UUO.