A method for diagnosing a pervasive developmental disorder, such as autism, comprising obtaining a sample of cerebrospinal fluid from a subject; obtaining a sample of serum from a subject; testing the cerebrospinal fluid of the subject for a concentration of TNF-.alpha.; testing the serum of the subject for a concentration of TNF-.alpha.; and positively diagnosing a pervasive developmental disorder when the concentration ratio of TNF-.alpha. in the cerebrospinal fluid of the subject to TNF-.alpha. in the serum of the subject exceeds approximately 2:1 over normal control concentrations.