Pulmonary tumor thrombotic microangiopathy (PTTM) is a cancer-related pulmonary complication characterized by rapid progression of dyspnea and pulmonary hypertension, occasionally causing sudden death. Given the condition of patients with dyspnea, lung biopsies are limited because of their invasiveness. A 72-year-old man presented with chronic atrial fibrillation and a high right heart load, as determined using ultrasonography. He had previously undergone resection of the left axillary skin secondary to extramammary Paget's disease (EMPD). Clinically, PTTM was suspected and pulmonary wedge aspiration cytology, collected from the pulmonary artery during catherization, was performed. Cytologically, the tumor demonstrated three-dimensional cell clusters with good cohesion and molding by the blood vessel lumen. Additionally, endothelial-like cells were observed at the periphery of the tumor clusters; fibrin was evident in the clusters. The tumor cells were large, round, and had high nuclear/cytoplasmic ratios. The nuclei demonstrated a variety of sizes and were irregularly shaped, with prominent nucleoli; cells undergoing mitosis were evident. The tumor cells were suspected of being poorly differentiated adenocarcinoma cells, consistent with PTTM. Two days after the aspiration cytology, the patient died and a pathological autopsy was performed. Histologically, the PTTM was determined to have caused the pulmonary hypertension and the primary PTTM site was apparently derived from the EMPD. For rapid diagnoses, an understanding of the tumor's cytological features is important and should contribute to early treatment intervention. Aspiration cytology, using pulmonary artery blood samples, during catherization is a useful tool for diagnosing PTTM.