To study the etiopathogenesis of secondary drug failure to treatment with oral antidiabetic agents in patients with non-insulin-dependent diabetes (NIDD) we compared 60 "nonresponders" with 60 "responders" to treatment with oral drugs. Secondary drug failure was defined as mean diurnal blood glucose greater than 12 mmol/L after an initial good response of greater than or equal to 2 yr. The nonresponders were characterized by 50% lower C-peptide concentrations than the responders (P less than 0.001). We could not, however, define a critical C-peptide level to discriminate between patients requiring and not requiring insulin therapy. There was a wide overlap of individual C-peptide values between responders and nonresponders that attenuates the clinical value of single C-peptide measurements in predicting therapy. Only by serial measurements over a period of time was it possible to achieve information about changes in beta cell function. The nonresponders showed increased frequency of islet cell (P less than 0.01), thyroid antimicrosomal (P less than 0.01), and gastric parietal cell antibodies (P less than 0.02). In nonresponders, HLA-antigen B8 was increased (P less than 0.05) and HLA-B7 decreased (P less than 0.01) compared with frequencies of responders. In conclusion, impaired beta cell function is a characteristic feature of many, but not all, NIDD patients who fail on treatment with oral antidiabetic drugs. The presence of islet cell and thyrogastric antibodies can unmask a distinct group of NIDD patients with a high risk of secondary drug failure and subsequent insulin dependency. HLA typing may further help to predict secondary failure in NIDD.