Mycoplasma pneumoniae is the second most common cause of community-acquired pneumonia. Infection is found worldwide and epidemics are said to occur in 4-yearly cycles. In Scotland this pattern has been noted since 1982 and, in common with England and Wales as well probably as other parts of Europe, there is a current epidemic which began in the autumn of 1990. The disease has been noted predominantly in children and young adults, with lower respiratory tract infection as the most common manifestation. At present, diagnosis is based on a serological response and various tests are available for detecting both primary infection and reinfection. In view of the present epidemic, initial treatment of respiratory-tract infection, especially in children and young adults, should include adequate cover against Mycoplasma pneumoniae. © 1992 The British Society for the Study of Infection.