Serum viscosity's increase in diabetes has been linked to the presence of microvascular sequelae and to changes in serum protein composition. The major change is a decline in albumin and an increase in the levels of acute-phase proteins. In this study, albumin and five acute phase proteins--alpha-1 acid glycoprotein, alpha-1 antitrypsin, haptoglobin, ceruloplasmin, and C-reactive protein--were measured. Levels in adult diabetes (principally type II) were compared with those in both subjects with glucose intolerance and control subjects (healthy subjects and nondiabetic ambulatory patients). Haptoglobin, alpha-1 acid glycoprotein, and C-reactive protein increased markedly in both diabetes and glucose intolerance; ceruloplasmin and alpha-1 antitrypsin increased more marginally. Serum albumin level decreased more strikingly as hyperglycemia advanced. Acute-phase proteins also increased in advanced glucose intolerance as in established diabetes. The acute-phase protein elevation did not differ with degree of control or duration of diabetes. When diabetics were divided into those with and without clinically detectable evidence of microvascular sequelae, elevation of haptoglobin, C-reactive protein and alpha-1 acid glycoprotein, and depression of albumin were found to progress with number of sequelae. The levels of these proteins, particularly haptoglobin, were also highly correlated with serum viscosity expressed as viscosity number. Mild serum albumin depression and a more striking acute-phase protein elevation are greater in diabetes with microangiopathy, develop in glucose intolerance, and contribute substantially to elevated plasma viscosity in diabetes.