Non-cirrhotic, non-malignant portal vein thrombosis (PVT) is commonly secondary to inherited or acquired prothrombotic states. However, even after extensive workup, 25% of patients with PVT have no apparent prothrombotic aetiology identified (idiopathic PVT). Inherited conditions include factor V Leiden, PT mutation and protein C/S/AT deficiency. Acquired conditions include APS, PNH and BCR-ABL 1-negative myeloproliferative neoplasms (MPN). BCR-ABL-1 negative MPNs are the most frequent underlying prothrombotic risk factor for PVT (15%-30%). However, peripheral blood counts often remain within normal ranges in these patients with MPN because of portal hypertension sequel. Despite suggestive features of MPN in bone marrow, these patients lack adequate diagnostic criteria and are classified as occult MPN. The discovery of recurrent molecular abnormalities such as CALR gene exon 9 mutation presented a crucial advance in the diagnosis of occult MPNs. In our patient, the diagnosis of MPN was made on this basis, despite lack of peripheral evidence of MPN.