Pulmonary alveolar proteinosis (PAP) is a rare disease characterized by the accumulation of lipoproteinaceous material in the alveolar space. Alveolar macrophages (AM) in this disease have been noted in the past to have abnormally decreased function. It is not known whether these abnormalities are reversible with treatment of the disease. In this study, the effect of therapeutic whole lung lavage (WLL) on AM migration in four patients with PAP was determined. Standard segmental bronchoalveolar lavage (BAL) was performed 1 day before and 6 days after WLL. AM recovered by BAL were assayed for migratory response to N-formyl-methionyl phenylalanine (FMP) and endotoxin-activated human serum (EAHS). As expected, migration toward both stimuli was decreased before WLL compared to a group of normal controls. After WLL, AM migration toward both stimuli was not only greater than before the WLL but also was greater than normal. In one patient, serial BAL over a period of 18 months showed a marked decline in AM migration coincident with a clinical relapse of the disease. Thus, the defect in AM function found in PAP is reversible with WLL but can recur if clinical remission is not sustained.