A 66-year-old woman presented with an unspecified retroperitoneal mass measuring ∼4.4 × 4.4 cm with concomitant hydronephrosis, ureterectasis, and a simple kidney cyst as confirmed by magnetic resonance imaging and computed tomography (CT) scans. The patient arrived to the center during the intervening 8-week period between the initial scans and the follow-up CT-guided biopsy. The visit was planned 3 months prior with the intention of improving her overall health. She elected to undergo a 13-day medically supervised, water-only fast followed by an 11-day refeeding protocol with a whole-plant-food diet free of added salt, oil, and sugar. Eighteen days after her departure, she had the follow-up CT-guided biopsy that showed the retroperitoneal mass had reduced to 2.8 × 1.8 cm, no longer requiring biopsy, and that the hydronephrosis and ureterectasis had also resolved. The time of these observations suggests a possible correlation between fasting and/or diet and the regression of unspecified masses as well as reduction of kidney and ureter inflammation.