We present the case of a 67-year-old man who presented to our hospital with acute shortness of breath. He was initially treated for a presumed infective exacerbation of chronic obstructive pulmonary disease (COPD) but did not respond to standard therapy. Further investigations were performed and revealed a large pulmonary vein thrombus. Clinical, laboratory, and radiological investigation did not identify an underlying cause for his thrombus. Idiopathic pulmonary vein thrombosis is much rarer than the more well-known pulmonary embolism and is worthy of consideration as an uncommon cause of pulmonary decline. To our knowledge, this is the first case report in the English literature of idiopathic pulmonary vein thrombus occurring in the setting of known COPD.