Lawsonia intracellularis is not culturable with a standard bacteriologic culture. One step PCR assay as a clinical diagnostic method was developed for the rapid detection of porcine proliferative enteritis (PPE) caused by L. intracellularis. Primers were designed based on the p78 DNA clone of L. intracellularis. The one step PCR resulted in the formation of a specific 210-bp DNA product derived from L. intracellularis. The nonspecific amplification product was not detected with swine genomic DNA or other bacterial strains causing similar symptoms to L. intracellularis infection. The one step PCR was as sensitive as 100 pg of L. intracellularis genomic DNA. We applied this method to field specimens diagnosed as PPE by macroscopic observation. Of 17 mucosal scraping specimens, 16 (94%) were identified as positive to PPE and 15 (88%) of 17 feces specimens. These results suggest that the one step PCR can be used as a rapid diagnostic method for L. intracellularis infection.