Eight psychiatric criteria were tested for their usefulness in distinguishing between conversion reactions and organic symptoms. Three were found most useful and can form a predictive index. They include (1) the patient's prior use of physical symptoms as a psychological defense, (2) the presence of a significant emotional stress prior to the onset of the symptom, and (3) evidence that the symptom was being used to solve a conflict brought about by the precipitating stress. These criteria can serve as additional data for the diagnosis of conversion reaction, and are useful in ruling out conversion reactions thus reopening the search for organic illness. The diagnosis of "hysteria" and the presence of a model for the symptom confirmed a conversion reaction. The presence of an hysterical personality, "la belle indifference," and the expression of symbolism in the symptom were not helpful.