Nephrocalcinosis is a chronic tubulointerstitial nephropathy characterized by tubular calcium phosphate deposition and slowly progressive renal insufficiency. We report a novel association of acute nephrocalcinosis and acute renal failure (ARF) with colonoscopy preceded by a bowel-cleansing regimen consisting of oral sodium phosphate solution (OSPS). A cohort of 5 patients (mean age, 69.2 years) had normal renal function (mean serum creatininem 0.9 mg/dL) before colonoscopy and presented with ARF (mean serum creatinine, 4.9 mg/dL) from 3 days to 2 months postcolonoscopy. Past medical history included hypertension in all 5 patients. Medications included an angiotensin-converting enzyme (ACE) inhibitor (ACE-I) or angiotensin receptor blocker (ARB) in 4 patients and diuretics in 2 patients. In all patients, colonoscopy was preceded by bowel cleansing with OSPS; OSPS was contraindicated in a single patient with hyperparathyroidism and was used at excessive doses in another. Renal biopsy specimens obtained from all 5 patients revealed diffuse tubular injury and abundant tubular deposition of calcium phosphate. Although the tubular injury involved all tubular segments, lectin and immunohistochemical staining disclosed calcium phosphate deposition confined to distal tubules and collecting ducts. At a mean of 5.8 weeks of postbiopsy follow-up, renal function was unchanged in 4 patients and mildly improved in 1 patient. We conclude that acute nephrocalcinosis is a seemingly rare complication of bowel cleansing with OSPS. The pathophysiology of acute nephrocalcinosis after treatment with OSPS likely involves transient hyperphosphatemia; volume depletion exacerbated by intercurrent ACE-I, ARB, and diuretic use; and elevated distal tubular phosphate and calcium concentrations. Greater awareness of this entity is needed to identify potential risk factors.