The infection levels of Mycoplasma hyorhinis, M.hyopneumoniae and M.hyosynoviae in the lung of piglets were examined in relation to porcine reproductive and respiratory syndrome (PRRS). These animals consisted of 43 PRRS piglets with PRRS, 2 piglets infected with PRRS virus but symptom-free, and 10 control piglets free of PRRS virus and its antibody. M.hyorhinis was isolated from 40 of the 43 PRRS piglets, from 1 of the 2 latent infected piglets and from 3 of the 10 control piglets. The number of M.hyorhinis isolated from the lungs of PRRS piglets was more than 10⁵ CFU/g, but those isolated from the latent infected piglets and the control piglets were less than 10³ CFU/g. In addition to this, Haemophilus parasuis and Pasteurella spp. were frequently isolated from the piglets with PRRS (51.2% and 25.6%, respectively). On the other hand, M.hyopneumoniae was isolated from only 4 of 55 piglets tested, and M.hyosynoviae was not isolated. M.hyorhinis was also detected directly in the lung emulsion samples from almost all the PRRS piglets using a polymerase chain reaction-based method.