Plasma fibronectin and serum thyroid parameters were determined in 6 hyperglycemic nonketoacidotic patients (HNK) and 12 subjects with diabetic ketoacidosis (DKA). The DKA patients showed a marked increase in both plasma fibronectin and serum T3 over 5 days of insulin treatment [175.2 +/- 18.1% (+/- SEM) and 208.7 +/- 17.6% of initial values respectively], while these parameters did not change in the HNK patients despite equivalent control of diabetes. Serum rT3 levels declined, as expected, to 65.8 +/- 10.9% of the initial values in the DKA patients, but did not change in the HNK patients. There was a significant positive correlation between changes in plasma fibronectin and serum T3 values in the DKA patients (r = 0.5; P less than 0.005). Other reports have shown a decrease in plasma fibronectin concentrations in fasted patients, a well known low T3 state; therefore, the association between changes in plasma fibronectin and serum T3 values may be a widely observed phenomenon. The parallel changes in fibronectin and T3 may reflect alterations in the metabolic state of these patients. The precise nature of the relationship between changes in fibronectin and T3 concentrations requires additional investigations.