Iron deficiency is a common cause of reactive thrombocytosis resulting in usually mild to moderately increased but sometimes even in extreme thrombocytosis (ie, >1000×109/L). We report a case of a 34-year-old woman who developed an increased platelet count of 1953×109/L. Upon admission, cytoreductive therapy was initiated until an underlying chronic myeloproliferative neoplasia was ruled out. The patient had undergone bariatric surgery 5 years previously, and surprisingly, a diagnosis of reactive thrombocytosis due to iron deficiency secondary to iron malabsorption was made. It is concluded that the degree of extreme thrombocytosis may be even fourfold to fivefold increased in patients with severe iron deficiency. Our finding emphasises the importance of regular control of possible need for iron supplementation following bariatric surgery.