Stool osmolality and electrolyte measurements were obtained from 12 patients with diarrheal disorders. Osmolality of diarrheal stool (285 to 330 mosmol) regardless of the cause is less than the reported osmolality of normal stool. Storage of stool at room temperature can artifactually increase stool osmolality as the result of bacterial metabolism. When stool samples are fresh, a negative osmotic gap (measured osmolality - 2 X [Na + K]) is commonly associated with secretory diarrhea, whereas a high osmotic gap (greater than 160 mosmol) is seen in patients with osmotic diarrhea. In many conditions fasting does not resolve diarrhea completely, and when the stool osmotic gap is greater than 50 mosmol, the pathogenesis of diarrhea is difficult to define.