The occurrence of increased circulating immune complexes (CIC) in sera of patients with esophageal cancer and their usefulness for diagnosis and prognosis have not been demonstrated. Circulating acute-phase proteins (APP) related to esophageal cancer have been described but without any association with CIC. This is a study to measure CIC, C-reactive protein (CRP), and alpha 1-acidic glycoprotein (AAG) in pretreatment esophageal cancer sera and to analyze the presence of both APP associated with these CIC. Increased CIC levels were found in 57% of sera from esophageal cancer patients; elevated CRP was detected in 87% and AAG in 47%. Western blot analysis showed the presence of CRP and AAG in CIC-derived fractions. We conclude that: (1) CIC, CRP, and AAG are elevated in esophageal cancer sera; (2) they may be considered possible useful clinical parameters in pretreatment esophageal cancer patients; (3) these APPs appear in CIC precipitates and may possibly be involved in their composition.