To the Editor.— The imprecise incident of incidence written about by Dr. Schoenberg (229:1724, 1974) is itself quite imprecise. In attempting to clarify an area fraught with confusion, he has unfortunately confounded the matter still further by using incidence and incidence rates synonymously and interchangeably. I offer the following example to help untangle the problem. If 3,500 new cases of malignant neoplastic disease occurred in Philadelphia in 1974, the incidence of that condition would be 3,500. The incidence rate , on the other hand, is obtained by dividing 3,500 by 4,800,000, the population of Philadelphia, and multiplying the decimal value by 100,000. In so doing, we obtain an incidence rate of 70. The incidence of 3,500 and incidence rate of 70/100,000, while reflecting the same condition, have different meanings.