This study analyzed the reliability of the final diagnosis in the 155,242 dengue reports during the 2001-2002 epidemic in the city of Rio de Janeiro, Brazil, using the official information system on communicable diseases (SINAN). The system allows the following options for the final diagnosis: classic dengue, dengue hemorrhagic fever, discarded, inconclusive, and unknown. We built a classification routine in Epi Info to compare the final diagnosis from SINAN with Ministry of Health criteria. According to the final diagnosis, the case breakdown was: 52.4% classic dengue; 0.6% dengue hemorrhagic fever; 0.9% discarded; 46% inconclusive and unknown. The revised diagnosis showed that 78% of classic dengue, 69% of dengue hemorrhagic fever, and 21.1% of discarded cases met the classification criteria. Although the reliability of the SINAN final diagnosis was generally satisfactory (kappa = 0.681; 95%CI: 0.685-0.677), it was worse for fatal cases (kappa = 0.152; 95%CI: 0.046-0.258). Considering the epidemic's magnitude, the final diagnosis of classic dengue and dengue hemorrhagic fever was satisfactory, but the high proportion of inconclusive or unknown cases and the poor quality of information for fatal cases limit the usefulness of SINAN in this context.