Ovine chlamydiosis, also known as enzootic abortion of ewes (EAE) or ovine enzootic abortion (OEA), is caused by the bacterium Chlamydia abortus. Chlamydial abortion typically occurs in the last 2–3 weeks of pregnancy with the appearance of stillborn lambs and inflamed placentas. However, infection can also result in the delivery of full-term stillborn lambs or weak lambs that do not survive longer than 48 hours. Infected ewes can also give birth to healthy lambs. There are rarely any predictive signs that abortion is going to occur, although behavioural changes and a vulval discharge can be observed in the last 48 hours of pregnancy. Chlamydial abortion also occurs to a similar extent in goats and, less frequently, cattle, pigs, horses and wild ruminants may be affected. In sheep, abortion in late pregnancy with expulsion of necrotic fetal membranes is a diagnostic indicator. Human infection may be acquired from infected products of abortion or parturition or from carelessly handled laboratory cultures of the organism, with manifestations ranging from subclinical infection to acute influenza-like illness. Authenticated cases of human placentitis and abortion caused by C. abortus of ovine/caprine origin indicate that pregnant women are at special risk and should not be exposed to sources of infection. Inactivated and live vaccines are available that have been reported to prevent abortion and to reduce excretion. They assist in control of the disease but will not eradicate it.