A dengue type 1 outbreak started in the Ribeirao Preto Region, North of Sao Paulo State, Brazil, in November of 1990. About 3500 dengue cases were confirmed by blood tests until February of 1991. The Virus Research Unit of The Faculty of Medicine of Ribeirao Preto - Sao Paulo State University, studied 502 dengue suspect cases. The Serologic diagnosis of dengue type 1 was confirmed by haemmaglutination inhibition test (HAI) in 19% of the cases. Diagnosis was done later by using an enzyme immuno assay on infected cultured cells (EIA-ICC) which discriminated IgG and IgM dengue, antibodies. EIA-ICC was less sensitive (89%) but more effective than HAI. EIA-ICC is a simple technique. It dispenses a second serum sample for diagnosis and it can be completed in about 5 hours. Dengue virus was isolated from the blood of 21 patients by innoculation in culture of mosquito C6/36 cells. The isolated virus were identified by indirect immunofluorescent test, by using an antisera pool to the flavivirus family and dengue type specific monoclonal antibodies. The dengue most frequent symptoms in 71 patients were observed: fever (90%), myalgias (57%) and arthralgias(41%)