A 5-day-old thoroughbred colt (foal 1) was referred for evaluation of progressive depression and abdominal distension since birth. Gestation length and parturition were normal, and CBC and immunoglobulin G (IgG) testing at 24 hours of age revealed an adequate IgG concentration (Ͼ800 mg/dL) and a normal white blood cell count (6,000 cells/L) (reference range, 5,500-12,500 cells/L). Diarrhea developed within 24 hours of birth and was treated with nonsteroidal anti-inflammatory drugs and metronidazole. The diarrhea resolved without identification of the etiologic agent, but the colt remained depressed. Abdominal ultrasonography by the referring veterinarian revealed a small bladder and an increased amount of peritoneal fluid of unusual echogenicity. No other abnormalities were noted during this examination. Abdominocentesis yielded serosanguinous fluid. No cytologic evaluation was performed on this sample. Initial physical examination at referral revealed a quiet colt in good body condition with a rectal temperature of 102.3ЊF (39.1ЊC), a heart rate (HR) of 100 beats/min, and a respiratory rate (RR) of 48 breaths/min. Weight was 153 lb (69.5 kg). Mucus membranes were pink, dehydration was estimated to be 5%, and abdominal distension was evident. Initial CBC, arterial blood gas, and clinical chemistry abnormalities included increased serum total protein concentration, hyponatremia, hypochloremia, hypoxemia, leukopenia (1,800 cells/L), neutropenia (468 cells/L) (reference range, 2,700-6,700 cells/L), hyperfibrinogenemia (816 mg/dL) (reference range, 150-300 mg/ dL), and azotemia. Serum IgG a was Ͼ800 mg/dL. Abdominocentesis fluid had an increased red blood cell count (84,000 cells/L) (reference range, 0-50,000 cells/L), an increased white blood cell count (3,740 cells/L) (reference range, 0-2,500 cells/L), and a normal total protein concentration (0.3 g/L) (reference range, 0-1.5 g/L) with no cytologic abnormalities. Peritoneal fluid creatinine concentration was 6.6 mg/dL, and the peritoneal fluid : serum creatinine ratio was 2.4 : 1. Ultrasonographic examination of the abdomen revealed an urachal defect, an intact bladder, and a large volume of echogenic fluid in the abdomen. Electrolyte disturbances of hyponatremia and hypochloremia, in concert with an increased peritoneal fluid : serum