Aerosol exposure of eight pregnant sows to cell-culture- propagated Lelystad virus resulted in clinical signs characteristic of so-called mystery swine disease. After an incubation of 4-7 days, all sows were inappetant and listless for 6-9 days. Two sows developed a transient red-blue discolouration of the ears ('abortus blauw' or blue ear disease) accompanied by abdominal respiration, and two had a fever for one day only. One sow aborted at 109 days of gestation. The other seven sows, farrowing between 113 and 117 days of gestation, gave birth to numerous mummified, dead, and weak piglets. Of these seven, the mean number of piglets born dead to each sow was 4.6 and the mean number born alive was 7.7; 3.1 piglets per sow (40%) died within the first week. Lelystad virus was isolated from 31 piglets, which were born dead or died shortly after birth. Antibody was detected in precolostral blood samples or ascitic fluids of 23 piglets, a finding which demonstrated transplacental passage of the virus in six out of eight litters. We conclude that Lelystad virus is the causal agent of mystery swine disease. Since its aetiology is no longer a mystery, we propose the more appropriate name 'porcine epidemic abortion and respiratory syndrome (PEARS)'.