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Abstract Background Non-small cell lung cancer (NSCLC) is the leading cause of cancer-related deaths worldwide. In Ireland alone, there are over 2500 new cases of lung cancer diagnosed each year. It ranks fourth among the most common cancers and causes 21% of all cancer-related deaths. Lung cancers usually metastasize to the liver, brain, bone, and adrenal glands—rarely affecting the abdomen. To our knowledge, there are 8 previous studies in the literature to date which involve omental metastasis from NSCLC. Case presentation A 73-year-old lady presented with a history of productive cough for 18 months. A chest X-ray showed a suspicious 6 cm mass in the right lower zone. She went on to have an endobronchial biopsy which confirmed a moderately differentiated adenocarcinoma of the lung which was ALK positive. She went on to have staging PET and CT scans and was staged as cT3N2M0. She was not a suitable candidate for surgery so she had radical chemo-radiotherapy with 4 cycles of cisplatin pemetrexed followed by radical dose sequential radiotherapy. Post treatment CT showed the tumor to be more spiculated in appearance. She was ineligible for Durvalumab maintenance therapy due to the extensive pneumonitis following her radiotherapy which required a prolonged course of steroids. Interval scans every 3 months did not show any progression of disease. She presented to the hospital 13 months post her diagnosis with progressive abdominal swelling. Restaging CT scans showed extensive omental infiltration, the presence of multiple peritoneal nodules, and progression of her pulmonary disease with new brain metastasis. Cytological analysis of the ascitic fluid confirmed metastasis followed by omental biopsy which confirmed metastatic ALK-positive NSCLC. She went on to have targeted treatment with alectinib. She tolerated the treatment well. Restaging scans done 3 months later showed good partial response to therapy. Conclusion In conclusion, ALK-positive NSCLC with metastasis to the omentum is very rare. However, in patients with atypical symptoms like ascites, the possibility of a metastasis must be considered and repeat biopsy is always recommended. A targeted therapy in the selected patients has shown a more durable response than chemotherapy.