In March of 1991, a disease that affected pregnant sows and caused a high mortality in unweaned piglets was detected in Spain. Based on the clinical signs observed, mystery swine disease, which had been described recently in Germany, Holland and Belgium, was suspected. From the samples obtained from the affected farm, a filtrable agent (0.22 micron) was isolated on cell culture. It produced cytopathic effects, its replication was intracytoplasmic, it was sensitive to chloroform, and cross-reacted with a Lelystad reference serum. When inoculated into pregnant sows, the agent produced inappetence for 2-4 days, without hyperthermia. One of the sows aborted at 100 days of gestation; the two others had delayed parturitions (days 115 and 116). There was a mixture of healthy piglets, mummified fetuses, stillbirths and weak piglets. Microscopic examination of the lungs of healthy piglets killed at 8 and 12 days of life revealed the presence of interstitial pneumonia. The sera from the three sows at 39 days after infection cross-reacted with the Lelystad virus (titres > or = 1/640), whereas pre-inoculation sera did not recognize it (titres < or = 1/10). This is the first report from Spain of the isolation of an agent (antigenically related to the Lelystad virus), capable of reproducing the disease previously designated as mystery swine disease.