Between January 2007 and April 2009, rotavirus (RV)-positive stool samples from 238 children with acute gastroenteritis, seen at Children's Hospital of Michigan in Detroit, USA, were collected and RV genotyping was performed. G and P genotypes were determined by RT-PCR and nucleotide sequencing was conducted on selected G9 and P strains. Correlation between the severity of gastroenteritis episode and the infecting G genotype was done using a 14-point scoring system. The predominant G genotype was G9 (39.5%), followed by G1 (35.3%) and G4 (15.5%), while P was the most prevalent P genotype (66.5%), followed by P (21.9%) and P (11.2%). The gene combinations G1P and G9P were the most prevalent (21.4% and 20.6%, respectively), followed by G4P (13%) and G9P (8.8%). Immunization data showed that only 17/238 (7.1%) children received ¢one dose of RV vaccine (the pentavalent vaccine RotaTeq or the monovalent vaccine Rotarix) and that 10/17 were infected with G4P strains. Severity of RV gastroenteritis episodes was not related to the infecting G genotype. Our results suggest a high proportion of genotype G9 strains in combination with P, P and P specificity circulating in the metropolitan Detroit area. While the protective efficacy of the RV vaccines has been demonstrated against G9P strains, the level of cross-protection offered by the vaccines against G9 strains with P and P genotypes in the Detroit paediatric population remains to be determined.