To reinvestigate the viral hypothesis of schizophrenia as well as possible immunological dysfunction, neopterin, which is an indicator of the activity in the cellular immune system, was determined in acute and chronic schizophrenics. Both diagnostic groups showed neopterin levels within the normal range. Patients with chronic schizophrenia of the residual type presented a significant dependency between neopterin concentrations and Brief Psychiatric Rating Scale (BPRS) total scores. In a long-term observation of 14 acute schizophrenic (paranoid type) inpatients, neopterin levels were found to be lowest at day 0. During the first week of treatment neopterin concentrations increased significantly whereas a concomitant decrease of the psychopathological symptoms could be observed. In comparison to healthy controls patients showed significantly lower neopterin levels at baseline. These findings will be discussed referring to the viral hypothesis of schizophrenia as well as to possible immunological alterations caused by stress or by changes in neurotransmitter synthesis.