AIM:To clarify human papillomavirus (HPV) involvement in carcinogenesis of the upper digestive tract of virological and pathological analyses. METHODS:The present study examined the presence of HPV in squamous cell carcinomas of the oral cavity (n = 71), and esophagus (n = 166) collected from Japan, Pakistan and Colombia, with different HPV exposure risk and genetic backgrounds.The viral load and physical status of HPV16 and HPV16-E6 variants were examined.Comparison of p53 and p16 INK4a expression in HPV-positive and HPV-negative cases was also made.RESULTS: HPV16 was found in 39 (55%) oral carcinomas (OCs) and 24 (14%) esophageal carcinomas (ECs).This site-specific difference in HPV detection between OCs and ECs was statistically significant (P < 0.001).There was a significant difference in the geographical distribution of HPV16-E6 variants.Multiple infections of different HPV types were found in 13 ECs, but multiple infections were not found in OCs.This difference was statistically significant (P = 0.001).The geometric means (95% confidence interval) of HPV16 viral load in OCs and ECs were 0.06 (0.02-0.18) and 0.12 (0.05-0.27) copies per cell, respectively.The expression of p16INK4a proteins was increased by the presence of HPV in ECs (53% and 33% in HPV-positive and -negative ECs, respectively; P = 0.036), and the high-risk type of the HPV genome was not detected in surrounding normal esophageal mucosa of HPV-positive ECs.CONCLUSION: Based on our results, we cannot deny the possibility of HPV16 involvement in the carcinogenesis of the esophagus.