Diffuse pulmonary lymphatic diseases are typically diagnosed shortly after birth or in childhood, but rarely may become evident in adulthood. We report 3 adult patients who presented with diffuse interstitial lung disease clinically and radiologically but on biopsy were found to have diffuse pulmonary lymphatic disease (2 cases of pulmonary lymphangiectasis and 1 case of pulmonary lymphangiomatosis). These patients presented with the insidious onset of symptoms including shortness of breath and cough. Imaging studies of the chest showed diffuse pulmonary interstitial opacities, often with a perilymphatic distribution. The clinical differential diagnostic considerations before surgical lung biopsy included infection, neoplasm, and interstitial lung disease. The histopathologic features included abnormal vessels and associated fibrosis following lymphatic routes, namely visceral pleura, bronchovascular bundles, and interlobular septa. Lymphangiectasis was characterized by dilation of normally distributed lymphatic spaces, whereas lymphangiomatosis showed a complex anastamosing proliferation of lymphatic vascular spaces without significant dilatation. The dilated lymphatic spaces often had undergone muscularization, which could easily lead to misclassification as veins. Immunohistochemical staining for the lymphatic endothelial marker D2-40 was helpful in correctly classifying these lesions. Diffuse pulmonary lymphatic disease can rarely present in adulthood, wherein the histologic findings can be subtle and could be overlooked as nonspecific reactive changes or misdiagnosed as an idiopathic interstitial lung disease. Recognition of the characteristic lymphangitic distribution of abnormally dilated or reduplicated lymphatic spaces is key to the correct diagnosis.