Six hundred and forty-six patients with an eosinophilic pleocytosis were studied in Thailand over a 3-year period ending in March 1968. It was possible to segregate the cases into two clinical entities, which were termed typical eosinophilic meningitis and eosinophilic myeloencephalitis respectively. The clinical and laboratory finding of 484 cases of typical eosinophilic meningitis are reported here. The latter entity was generally benign and self-limited with a case mortality ratio of less than 0.5%. Acute severe headache was the most significant symptom. Fever was uncommon and abnormal neurologic findings were absent in 58% of cases. Sixteen percent of patients had visual impairment and 12% had an abnormal fundus. Impairment of the sensorium of a slowly progressive type and weakness of the extremities without localization were noted in 5% and less than 1% of patients, respectively. These signs occurred only in severely ill patients. Involvement of the cranial nerves was found in 17% of patients. Paralysis of the external rectus muscle of the eye and facial paralysis were found in 3% and 4%, respectively. The cerebrospinal fluid was characteristically turbid with a leucocyte count of more than 500 cells per mm3 in 75% of cases. It appeared that specific treatment of the disease was not indicated and that steroids and antibiotics did not have a definite beneficial effect on the course of the illness.