In contrast to the substantial evidence for attenuation of the glomerular lesions by a low-protein (LP) diet, it remains to be determined whether and how such a diet lessens the progression of tubulointerstitial lesions, which show the strongest correlation with renal function. Chronic unilateral ureteral obstruction (UUO) results in interstitial fibrosis of the affected kidney. We investigated the therapeutic effects of an LP diet on the progression of interstitial fibrosis in UUO mice. Sixty ICR mice underwent UUO or sham operation; half of these mice were fed a normal-protein (NP) and the other half a LP diet. They were sacrificed at 3, 7 and 14 days postoperatively. The degree of tubular lesion, the distribution of transforming growth factor-β (TGF-β), α-smooth muscle actin and fibronectin and the activated TGF-β1 level were determined. The LP diet significantly reduced the progression of tubular injury, depositions of fibronectin, tubulointerstitial myofibroblast formation, the interstitial expression of TGF-β-positive cells (at 14 days; NP = 6.91 ± 3.35 vs. LP = 1.67 ± 0.41; p < 0.005), and renal active TGF-β1 concentration (at 14 days; NP = 5.72 ± 2.03 vs. LP = 2.96 ± 0.72; p < 0.01). We conclude that protein restriction may aid the attenuation of progression of tubulointerstitial fibrosis through the reduction in tubulointerstitial expression of TGF-β.