A total of 489 Vibrio parahaemolyticus isolates from patients in Thailand with diarrhea was examined for the presence of thermostable direct hemolysin (TDH) and TDH-related hemolysin genes (tdh and trh, respectively), their serovars, TDH production, and urease activity. Of the strains, 81% were positive only for the tdh gene, 6% for both trh and tdh genes, and 2% for the trh gene only. Thirty-seven (8%) of the 489 isolates were positive for urease production. Of special interest, all urease-positive strains possessed the trh gene, and conversely, urease-negative strains lacked the gene, indicating that urease production by V. parahaemolyticus strains strongly correlates with the possession of the trh gene. Thus, the urease-positive phenotype of V. parahaemolyticus can be considered an indication of virulent (trh-possessing) V. parahaemolyticus strains in clinical diagnosis.