This longitudinal study analyzed how the activation of cell-mediated immunity (CMI) was related to the severity of symptomatology in 25 acute schizophrenic inpatients (DSM-III-R, 295.31). Neopterin, which was used to monitor the activation of T-cells and macrophages, was found to be within the normal range, but the lowest neopterin concentrations were measured on day 0. By day 3, a significant increase of neopterin was observed. Compared with healthy controls, patients had significantly lower neopterin levels at baseline. The highest scores on the Clinical Global Impressions Scale and the Brief Psychiatric Rating Scale occurred on day 0 and decreased significantly over the observation period. In general, the increase of neopterin was accompanied by a decrease in psychopathological symptoms. These results' indicate that at study entry, when patients are acutely ill, activation of the CMI is reduced rather than increased. Possible pathophysiological mechanisms are discussed.