This work describes four patients with poly myalgia arteritica (PMA) and one patient with an illness compatible with PMA within one family. Of these 5 patients, 4 are siblings while one is a genetically unrelated husband. The husband had a classical GCA in 1969. Over a period of one year starting in 1975, the husband's wife and three of her siblings were affected with PMA. This observation is highly suggestive of an infectious agent as being responsible for PMA, and a genetic disposition is probably essential for development of the disease. The incubation period is probably in the range of 5–7 years.