Enrique de Areilza Arregui was born in Bilbao. He studied medicine in Valladolid and obtained his doctorate in Madrid. He was hired as the first director of the new mining hospitals of Triano, in Gallarta and La Arboleda, at the age of 22, by the Royal Academy of Medicine. In Gallarta, miners injured in work-related accidents filled the hospital wards, with thousands of operations, mainly for pelvis and skull fractures. He was one of the first surgeons to perform cranial trepanations using modern techniques, going on to achieve great prestige.
He travelled around Europe to study new techniques with a view to making his hospital among the best. Paris was decisive in his eagerness to learn. At the head of his team, he managed to bring the alarming worker mortality rate under control and to improve food and hygiene conditions, alerting people to the social nature of the problem. In 1885, he organised the campaign in Bizkaia against the cholera epidemic that ravaged the mining population.
In 1898-1899, he presided over the Academy of Medical Sciences of Bilbao, with which he had collaborated since it was founded in 1895. His medical prestige and unselfishness earned him great fame among the working class. In 1900, he opened the clinic called "Sanatorio Bilbaino".
Concerned about the ravages of bone tuberculosis among children, in1909 he obtained approval from the Provincial Council for a project to build a children's tuberculosis prevention centre. At the head of a commission of doctors, he visited the most important European medical centres in his field. After visiting the beaches of Bizkaia, he decided to build it in Gorliz. The plans, by provincial architect Mario Camiña, were approved in 1910. The project was awarded a prize at the "Esposicione Internazionale d'Igiene Sociale" in Rome in 1912. The Gorliz Marine and Heliotherapy Sanatorium was inaugurated in June 1919.
He chaired the first Board of the Medical Association of Bizkaia in 1917, and was responsible for drawing up its regulations. He carried out a study of the consequences of the influenza epidemic of 1918 on the population, the year in which he accepted the position of director of the Civil Hospital of Basurto.
The Provincial Government approached him about commissioning a project for which he led a study trip around the continent to collect advanced techniques, setting up the "Escuela de lisiados y tullidos" (School for the Crippled and Disabled) in 1926.
He died in Bilbao, after a brief illness, on 14 June 1926. The prestige and respect he enjoyed are attested to by various obituary articles. In 1926, a monument was erected in his memory, the work of Moisés de Huerta, in front of the Gorliz Sanatorium. And one of Bilbao’s most important streets bears his name.