Plasma concentrations of fibronectin, free insulin, C-peptide and plasma glucose were determined in 40 morbidly obese subjects and in 51 normal weight controls, matched for sex and age. All plasma concentrations were significantly elevated (p less than 0.01) among the obese subjects. A significant correlation (r = 0.34, p less than 0.05) between plasma fibronectin and plasma free insulin was found among the obese patients, but not among the controls (r = -0.02, p greater than 0.05). No significant correlation was found between plasma fibronectin and plasma C-peptide, neither in the obese patients nor in the controls (obese r = 0.06, controls r = 0.02; p greater than 0.05). Plasma fibronectin was insignificantly correlated with body weight (obese r = 0.21, controls r = 0.15; p greater than 0.05) and percentage overweight (obese r = 0.27, controls r = 0.04; p greater than 0.05). The raised level of circulating insulin may in part explain the excess of plasma fibronectin obese subjects.