If the age specific prevalence rates for known and unrecognized diabetics are known, deduction can be made of the rates at which cases are moving in and out of these categories and, therefore, the average time spent in each category. For example, if between the age of forty and fifty the number of known diabetics in a population increased from 300 to 600, then the increase (300 over ten years) indicates that previously unrecognized diabetics have become recognized at an average rate of thirty per year. Similarly, if the number of unrecognized diabetics has in-creased from 400 to 900, the unrecognized group has been increasing at an average rate of fifty per year. If the rate at which death has been removing persons from each group is known, it is possible to form a theoretical model and calculate approximately how long a person is likely lo remain in the unrecognized category.

When the calculations are carried through, it appears that the average length of time spent in the unrecognized category is of the order of ten to twelve years. Unfortunately, some of the steps in the calculations are based on unknown factors, and in assuming arbitrary values for these factors, there will be some error introduced into the final estimate. Since the method of calculation may be of interest and value, however, the argument will be presented in detail.

When a new case of diabetes mellitus is diagnosed, it is not known how long the disease has been present. In this paper an attempt is made to deduce from epidemiological data the average length of time between development of impaired carbohydrate tolerance and diagnosis.