Background: Two clinical severity scales, the Vesikari scale and the Clark scale, are commonly used to assess the efficacy of rotavirus vaccines. The results obtained using the two severity scales differ significantly and hamper comparisons. The aim of this study was to compare the Clark and Vesikari scales and to determine whether modified classifications would provide a better correlation between the two scales. Methods: The severity of rotavirus gastroenteritis was assessed for each child using both the Vesikari and Clark scales. To make a statistical comparison between the two scales, the classifications were modified. Results: In total, 200 children with rotavirus gastroenteritis were evaluated. Of these, 57% were classified as severe by the Vesikari scale, and only 1.5% by the Clark scale (p < 0.001). When the Clark three-category scale was transformed into a two-category scale by merging mild and moderate categories as non-severe, a good correlation with the Vesikari scale still could not be found. Using the median of the severity scores as the severity threshold did not provide a better correlation between the two scales. Transforming the Vesikari two-category scale into a three-category scale by further subdividing the severe category into two parts, as moderate and severe (>= 16), provided a better correlation between the two severity scales, but still did not achieve a good level of agreement. Conclusions: The Clark and Vesikari scales differ significantly in the definition of severe gastroenteritis. Even the attempts at reclassifying the scales did not achieve a good correlation.