Background: In our preceding study, we assayed in a blind fashion the blood sera of young normal subjects and schizophrenic patients for levels of platelet autoantibodies (PAA). The recorded PAA titers of the schizophrenic patients were significantly higher than those of the normal subjects. This observation has lent support to this test being used as an objective evaluation of schizophrenia in young subjects in the future. In addition, this finding strongly suggested that the etiology of a distinct group of sufferers of this disorder could have originated from an autoimmune reaction against platelets which can, under certain conditions, cross-react with brain tissue. Aims: In the present study, PAA titers in the sera of adult schizophrenic patients and matched normal subjects were determined analogously to the preceding study. The effect of hospitalization and drug treatments on the apparent blood test scoring in adult subjects could thus be evaluated. Methods: A total of 46 schizophrenia patients (30 men and 16 women) aged 19–45 years (mean ± SD: 31.7 ± 8.0 years) with a minimum score of 60 on the Positive and Negative Symptom Scale and 43 healthy control subjects (22 men and 21 women) aged 21–44 years (mean ± SD: 31.9 ± 6.9 years) participated in the study. The blood titers of PAA were evaluated in a single-blind fashion using an optimized ELISA test scored by optical density (OD) units. A positive test was defined as a value above 1.3 OD units. Results: The titers of PAA in the group of schizophrenic patients (1.1 ± 0.55 OD units, range: 0.360–2.285 OD units) were significantly higher in comparison to those of the healthy control subjects (0.81 ± 0.37 OD units, range: 0.360–1.704 OD units; p = 0.004, two-tailed unpaired t-test). Significantly more schizophrenic patients showed a positive test (15 patients out of 46) than the control subjects (5 out of 43). However, significantly higher OD values of 1.55 ± 0.5 were recorded in the group of patients with less than 3 years of registered disease (n = 16, age 19–30 years), while in the group with 4–20 years of hospitalization (n = 30, age 24–45 years) the recorded OD values (0.85 ± 0.4 OD units) were practically indistinguishable from those of the control group. Conclusions: In the adult schizophrenic patients, the PAA blood test remains valid for patients who were hospitalized for less than 3 years. Drug treatment, length of disease and age can be assumed to reduce the PAA level considerably.