A study was made of 94 sets of twins born during 1975-8. Nine of these sets had not been diagnosed before labour started. Of the others, 75 were diagnosed as a result of clinical suspicion and 10 were diagnosed unexpectedly during the antenatal period, nine by ultrasonic examination. Thus, while ultrasonic examination has substantially reduced the incidence of undiagnosed twins, a fifth of all patients who had ultrasonography performed in the presence of a twin pregnancy were reported on at least one occasion to have a singleton pregnancy. Ultrasonography must be performed at least twice, therefore, before a multiple pregnancy can be confidently excluded.