Idiopathic CD4+ T-lymphocytopenia is defined as a depletion of CD4+ lymphocytes below 300/mm 3 in the absence of HIV infection or other known causes of immunodeficiency. Many infectious diseases have been reported to be associated with idiopathic CD4+ T-lymphocytopenia, and there have also been a few cases of mycobacterial infection in idiopathic CD4+ T-lymphocytopenia. Until now, it has been unclear as to whether CD4+ T-lymphocytopenia is a predisposing factor for or a consequence of the mycobacterial infection. Pulmonary alveolar proteinosis is an uncommon disease characterized by the intraalveolar deposition of amorphous granular material that stains positive with PAS, and its association with mycobacterial infection has rarely been reported. Recently, we experienced a previously healthy young man who had been diagnosed as idiopathic CD4+ T-lymphocytopenia with disseminated mycobacterium kansasii infection and pulmonary alveolar proteinosis, and report this case.