The 1999 WHO classification of adenocarcinoma of the lung and pleural tumors listed five rare variants of adenocarcinoma of the lung: well-differentiated fetal adenocarcinoma, colloid 'mucinous' adenocarcinoma, mucinous cystadenocarcinoma, signet ring adenocarcinoma and clear-cell adenocarcinoma. The International Association for the Study of Lung Cancer/American Thoracic Society/European Respiratory Society developed a multidisciplinary, international classification of lung adenocarcinoma that was published in the February 2011 issue of the journal of Thoracic Oncology. This most current classification lists four rare variants of invasive adenocarcinoma of the lung: invasive mucinous adenocarcinoma (formerly mucinous bronchioloalveolar carcinoma), colloid adenocarcinoma (retained and expanded), fetal adenocarcinoma (retained) and enteric adenocarcinoma (new). Signet ring adenocarcinoma and clear-cell adenocarcinoma were removed from the list of variants of adenocarcinoma of the lung. Mucinous cystadenocarcinoma was merged into colloid adenocarcinoma. The new 2011 classification also takes into consideration of the amount of tissue sample available according to the two major methods how the tumor is procured: resection specimens versus small biopsy/cytology. Rare variants of invasive adenocarcinoma of the lung will only now be classified from resection specimens where adequate architecture of tumor can be identified.