In this study, adenosine deaminase (ADA) levels of serum and cerebrospinal fluid (CSF) in a total 28 children (13 with bacterial meningitis, 5 with mumps meningoencephalitis and 10 with febrile convulsions) were determined. The comparisons between the serum values were insignificant (p greater than 0.05) but the CSF levels of the children with bacterial meningitis were higher than in the others (p less than 0.05). These findings suggest that serum ADA levels are not important in the diagnosis and differential diagnosis of these diseases. However, ADA levels of CSF may be useful in differentiating between bacterial and viral meningitis.