Oral glucose-tolerance test (O.G.T.T.) plasma sugar and insulin levels were measured in 118 newly diagnosed maturity-onset diabetic patients before and after treatment with diet alone for periods of 2 and 6 months. The results of glucose-tolerance tests carried out during treatment could be predicted from the initial test and the weight reduction between the tests. This prediction was not improved by the addition of further variables, including age, obesity, and plasma-insulin levels during the first test. The change in O.C.T.T. plasma-insulin between the first and second tests was predicted by the result of the initial tests, the improvement of glucose tolerance between the two tests, and the degree of weight reduction. 95% of the group achieved some improvement of glucose tolerance after 2 months of dietary treatment, and 59% of the group achieved adequate diabetic control by this time. It is concluded that treatment with diet alone should be the first-line management for patients with newly diagnosed maturity-onset diabetes mellitus.