The role of group C rotaviruses as a cause of diarrhea was examined among children <17 years of age admitted to a Hospital in a suburban area of Buenos Aires, Argentina between 1997 and 2003. A total of 1,579 fecal samples were screened for group A (RVA) and C (RVC) rotaviruses by two in-house ELISA methods at Quilmes University (UNQ-ELISA). Samples positive, doubtful and negative by RVC specific UNQ-ELISA (n = 246) were examined further for RVC by another in-house ELISA (CDC-ELISA), electron microscopy, RT-PCR, nested PCR, and Southern hybridization. Sensitivity, specificity, and predictive values for each test were determined. While the sensitivity was comparable for the nested PCR and CDC-ELISA methods (82.5%), the molecular methods were slightly more specific. Poorly preserved particles were often seen in fecal samples, suggesting that degradation of RNA could be a factor influencing the performance of molecular methods. The incidence of RVC was estimated to be 3% without apparent differences among seasons. RVC infected patients had a significantly (P < 0.001) higher median age (6 years vs. 1 year) than those with RVA infection. Sequence of the RVC VP7 gene from six Argentinean strains and sequences reported previously in different countries showed high nucleotide (94.4-99.9%) sequence identities, indicating a high degree of conservation for human RVC VP7 genes among strains collected on five continents over a period of 17 years. These findings indicate that RVC is a significant cause of diarrhea and it is necessary to develop simple and sensitive serological methods for its detection.