Idiopathic hyperammonemic encephalopathy is a rare complication of chemotherapy that has been described in patients with hematological malignancy, colon cancer, and hepatocellular carcinoma in the presence of noncirrhotic liver. The cause of this syndrome, which can be fatal, is not well understood. We present a case of rectal cancer without underlying liver dysfunction who suffered abrupt alteration in mental status after the first course of chemotherapy. Ammonia-lowering therapy was started immediately, without cessation of chemotherapy. For additional courses of chemotherapy, reduction to 75% of normal dosage was found to be safe and to produce satisfactory results.
