A widespread epidemic of Mycoplasma pneumoniae infection in the United Kingdom occurred over a 30-month period in 1970-72. A small epidemic may also have occurred in 1967-68. Though the incidence of M. pneumoniae infection is known to change from year to year in small communities, a varying epidemicity in the population of a country has not previously been demonstrated.As expected, lower respiratory tract infection was the most common manifestation of infection; and the highest attack rates were in children aged 5-9 years. Elderly persons appear to be affected uncommonly.