Eero Antero Mäntyranta (20 November 1937 – 29 December 2013[1]) was one of the most successful Finnish cross-country skiers.

His performance at the 1964 Winter Olympics earned him the nickname "Mister Seefeld", referring to the venue where the cross-country skiing and biathlon competitions took place.

Mäntyranta had primary familial and congenital polycythemia (PFCP) causing an increase in red blood cell mass and hemoglobin due to a mutation in the erythropoietin receptor (EPOR) gene, which was identified following a DNA study that found the mutation in a high proportion of members of his extended family, as reported in 1993.

[3] The elevated hematocrit caused by the condition increases the ability of the blood to transport oxygen; the EPOR mutation is speculated to have contributed to Mäntyranta's remarkable endurance.

[2][4] In 1972 Mäntyranta tested positive for amphetamine at a Finnish competition, becoming the first of his countrymen known to be caught doping.