Pulmonary alveolar proteinosis (PAP) is a rare disease in which alveoli are filled with lipoproteinaceous materials. We measured carcinoembryonic antigen (CEA) concentrations in bronchoalveolar lavage (BAL) fluids and sera from patients with PAP and from healthy volunteers (HV). Significantly increased CEA values were found in BAL fluids and sera from patients with PAP compared with those from HV. BAL fluid CEA values significantly correlated to serum CEA values in patients with PAP. Serum CEA values significantly correlated to serum lactate dehydrogenase activity and alveolar-arterial PO2 difference values in patients with PAP. Successive measurements of serum CEA showed that serum CEA values reflect the disease activity of PAP. The determination of serum CEA is useful for evaluating disease severity of PAP.