In a 10-year prospective study of 241 people with 'borderline diabetes' (impaired glucose tolerance) identified by screening of the Bedford adult population, 36 (15%) worsened to diabetes and 128 (53%) substantially improved their glucose tolerance. The major predictor of worsening to diabetes was the level of blood glucose at baseline. This was statistically significant (p less than 0.05), independent of other factors, both for deterioration in the first and in the second five years of observation. Body mass index, a measure of adiposity, did not predict worsening to diabetes during the first five years, but was an independent and significant predictor of worsening during the second five years (p less than 0.05). The apparent effect of adiposity was complex, for it was also significantly related to improvement in glucose tolerance during the 10-year follow-up. Persons with impaired glucose tolerance are a heterogeneous group and with present knowledge the ability to predict metabolic deterioration is limited.