The differential diagnosis of erythrocytosis is complex, involving a tailored algorithm. Congenital causes are rare and such patients commonly face a long journey looking for diagnosis. This diagnosis requires expertise and accessibility to modern diagnostic tools. We present the case of a young Swiss man with long-standing erythrocytosis of unknown origin and his family. The patient had an episode of malaise as he went skiing above 2,000 m altitude. In the blood gas analysis, p50 was low (16 mm Hg) and erythropoietin was normal. Using next-generation sequencing, a mutation in the hemoglobin subunit beta gene was found, a pathogenic variant known as hemoglobin Little Rock causing high oxygen affinity. Some family members also had unexplained erythrocytosis, therefore the mutational status of the family was analyzed, the grandmother and mother showed the presence of the same mutation. The use of modern technology finally offered a diagnosis to this family., (© 2023 The Author(s). Published by S. Karger AG, Basel.)