The incidence of dengue infection at King Chulalongkorn Memorial Hospital from 1997 to 2000 was reviewed from laboratory information. The highest incidence of the disease occurred from the rainy season to the winter. Most cases were aged below 15 years (87.89%) and no sexual preponderance was observed. The majority of cases were secondary infection (62.09%). Detection of HI titer is still useful for diagnosis although paired serum has to be taken. The time interval between sera should be at least 7 days for suspected primary infection but a shorter time interval can be considered for suspected secondary cases. Rapid and accurate detection of dengue IgM has become more useful because only single serum is required. A request for dengue IgM should be done after day 5 of illness. However, interpretation of the result should be done carefully according to the timing of serum collection.