Plasma cells are usually absent in normal bronchoalveolar lavage (BAL) fluids. Frequent plasma cells in BAL are always pathologically significant. They raise a rather narrow differential diagnosis, predominantly extrinsic allergic alveolitis and few other interstitial lung diseases. BAL is a key diagnostic procedure for certain lung diseases. Because plasma cells are uncommon in BAL fluids and usually associated with limited conditions compared to other more common interstitial lung diseases, the focus of cell count differentials in BAL fluids was on other mononuclear and granulocytic inflammatory cells. Recently, cases of IgG4 positive plasma cell-related lung diseases have been reported in tissue specimens. This may raise the importance of cytologically recognizing plasma cells in BAL fluids. We report a case of a 16-year-old boy with clinical and radiologic features of diffuse interstitial lung disease. His BAL cytology showed frequent plasma cells. Endobronchial biopsy showed substantial infiltration by plasma cells confirmed by immunohistochemistry. After excluding other lung diseases, the possibility of IgG4 plasma cell-related disease was raised. Blood tests showed elevated serum levels of IgG4. This case emphasizes the importance of including of IgG4 plasma cell-related lung disease as a possible differential diagnosis. This should prompt proper serologic tests for confirmation and appropriate management. With increasing reports of IgG4-related lung disease, cytopathologists should be aware of this possibility when examining BAL fluids. Measurement of IgG levels in BAL fluids and plasma cell immunophenotyping might be promising tools.