A study was conducted in 4 provinces (Ha Noi, Bac Ninh, Hung Yen and Hai Duong) aiming at identifying epidemiological characteristics of the enzootic pneumonia of pigs. As a result. the prevalence of the disease was found high in all the 4 studied provinces varying from 19.92 to 21.16 percent. Concerning the age of the infected pigs. the highest prevalence (36.57 percent) was found in the pigs from 9 to 14 month old, a lower prevalence (0.24 - 2.73 percent) was found in other pig categories. However, the acute disease and high mortality was found in the younger pigs. The disease was not observed in the boars. The disease beard the seasonal character and occurred mainly in February, March. April, October and December. The relation between the prevalence and the husbandry methods was not found. The application of the semi-nested PCR has preliminarily identified M. hyopneumoniae in 10 of 16 pathological samples.