The effect of thymoquinone (TQ), the main constituent of the Nigella sativa L. oil, on ifosfamide (IFO)-induced Fanconi syndrome (FS) and its antitumor activity were investigated in rats and mice, respectively. In rats, a daily injection of IFO (50 mg/kg per day, i.p.) for 5 days induced a FS characterized by wasting off glucose, electrolytes and organic acids, along with elevated serum creatinine and urea, as well as decreased creatinine clearance rate. Administration of TQ with the drinking water of rats, (5 mg/kg per day) for 5 days before and during IFO treatment, ameliorated the severity of IFO-induced renal damage. TQ significantly improved IFO-induced phosphaturia, glucosuria, elevated serum creatinine and urea, and significantly normalized creatinine clearance rate. Moreover, TQ significantly prevented IFO-induced renal glutathione (GSH) depletion and lipid peroxide accumulation. In mice bearing Ehrlich ascites carcinoma (EAC) xenograft, TQ (10 mg/kg per day) administered in drinking water significantly enhanced the antitumor effect of IFO (50 mg/kg per day, i.p. on days 1-4 and 15-18). Furthermore, mice treated with IFO in combination with TQ showed less body weight loss and mortality rate compared to IFO single therapy. These observations demonstrate that TQ may improve the therapeutic efficacy of IFO by decreasing IFO-induced nephrotoxicity and improving its antitumor activity.