In patients with acute abdominal pain, abdominal paracentesis and diagnostic peritoneal lavage often yield fluid samples for cytologic and biochemical evaluation. Cytology of the effusion from a patient with acute abdominal disease can be a crucial tool for the rapid diagnosis necessary for initiation of timely and appropriate therapy. Appropriate sample collection, handling, and preparation are essential to obtain an accurate diagnosis. Analysis of the fluid sample should include gross examination of the effusion, measurement of total nucleated cell count, packed red blood cell volume, and protein concentration, as well as examination for the presence of other cells, bacteria, food particles, or plant material. Biochemical evaluation should proceed based on the clinician's index of suspicion for a particular disease process. Abdominal effusions are generally classified as transudate, modified transudate, or exudate, depending on the total nucleated cell count and protein concentration. Cytology of all fluids collected should be performed systematically, utilizing progressively higher magnifications with a microscope. Specific diseases with associated abdominal effusions include septic peritonitis, nonseptic peritonitis, hemoabdomen, uroabdomen, pancreatitis, bile peritonitis, chylous effusion, and neoplasia. A complete description of sample preparation and evaluation is reviewed.