A 62-year-old man with a recent diagnosis of primary central nervous system lymphoma developed acute kidney injury (AKI) after administration of high-dose methotrexate. AKI developed within 48 h of therapy despite prior infusion of intravenous bicarbonate solution. Serum methotrexate level was 2.61 μmol/l at 24 h and 0.25 μmol/l 72 h after methotrexate administration. Urinalysis was negative for hematuria, proteinuria, or pyuria. Urine microscopy revealed amorphous brown precipitates (Figure 1). To determine whether these findings were consistent with methotrexate precipitation, we added sodium methotrexate solution to a buffered acidic solution (pH 4.7). The resulting precipitates (Figure 2) were similar in color and morphology to those observed in the patient's urine. Urine methotrexate level was 4.9 μmol/l as measured by fluorescence polarization immunoassay. Methotrexate-induced AKI is attributed to tubular obstruction due to precipitation of methotrexate, which may be exacerbated by low urinary pH and reduced tubular flow due to hypovolemia. To our knowledge, this is the first case of urinary methotrexate precipitates in the medical literature.