In a control scheme for enzootic-pneumonia-free herds, run by the Pig Health Control Association, a detailed study was made of 55 herds that developed enzootic pneumonia without a simple explanation. These herds were compared with 57 herds that were still free from enzootic pneumonia in mid-1984. A high standard of precautions against the risk of infection being transferred by people and fomites seemed to confer no obvious benefit. This observation was in keeping with in vitro studies which showed that, although Mycoplasma hyopneumoniae could survive for a long time in favourable liquid medium, it could not be recovered from material such as cloth, once the culture had become dry. Under field conditions, the organism would probably cease to be infective within 48 hours. The organism survived particularly well in rain water at lower temperatures, however, and transmission via moist cold air seemed a possibility. There was a tendency for breakdowns to start in the autumn and winter, particularly in highly secure units, and several farmers associated colder misty conditions with the arrival of infection. One herd was probably infected by an imported boar and the very close proximity of foreign pigs, such as in slaughterhouse transport, seemed the most likely explanation in 15 other herds. One herd was replaced without this danger being attended to and it soon broke down again, whereas the three herds in this category that have survived after replacement all had this risk eliminated. Data was available on 37 of the 39 remaining herds to compare them with the 57 surviving herds, using a risk index based on the proximity of other pig units.(ABSTRACT TRUNCATED AT 250 WORDS)