During the dengue epidemic from late 1987 to 1989, 6 specimens of cerebrospinal fluid (CSF) and sera for IgM detection were collected from 4 cases virologically confirmed dengue patients who had neural symptoms. Another 20 serum specimens, which had been diagnosed as dengue infection either virologically or serologically, were sent to the laboratory from Kaohsiung Medical College Hospital. All these specimens were also taken to detect the existence of IgM. The results showed that IgM could be detected from 14 out of 20 serum specimens. One of the positive specimens showed IgM can last up to 252 days after onset of illness. In addition, IgM was detected from both CSF and sera of all four dengue patients with neural symptoms. The IgM titer in CSF (less than or equal to 1:20) was always lower than that in serum (greater than or equal to 1:80). Two cases with sequentially collected specimens showed the fading of IgM titer in CSF. As a matter of fact, it became undetectable about a month after onset of illness, which is apparently different from the situation in serum.