The aim of this work was to ascertain whether spatial clustering of twin deliveries occurred in Sweden between 1973 and 1990. Since 1973, the Medical Birth Registry (MBR) has gathered information on all births in Sweden. Between 1973 and 1990, there were 17,067 twin deliveries. The MBR records data on pregnancies, deliveries, and the neonatal period, including parturients' domicile. To test the hypothesis that atmospheric pollution from refuse incinerators increases the incidence of twin deliveries, the periods before and after the commissioning of 14 such plants were compared. There was no apparent increase in the number of parishes or municipalities having a relative excess of twin deliveries. In the vicinity of one plant, there was a significantly increased number of twin deliveries, whereas in another, there was a significant decrease. There was no spatial clustering when the incidences of twin delivery were compared among parishes, municipalities, and areas near incinerators.