We then examined the correlation between serum total serine, L-serine, D-serine levels and BPRS scores of patients with schizophrenia. Interestingly, there was a significant positive correlation between serum D-serine levels and total scores (r = 0.542, P = .003), positive symptom scores(r = 0.589, P<.001), and negative symptom scores (r = 0.427, P = .02) on the BPRS among the medicated patients, whereas no such correlation between serum total serine or L-serine levels and total or each subscale score on the BPRS was detected. Because these correlations might have been confounded by medication, we controlled for the dose of antipsychotics using partial correlation coefficients. Even when the administered antipsychotics (chlorpromazine equivalents) were adjusted for using partial correlation coefficients, the relationship among serum D-serine levels and total scores (partial correlation coefficient = 0.488, P = .01), positive symptom scores (partial correlation coefficient = 0.545, P = .004), and negative symptom scores (partial correlation coefficient= 0.401, P = .04) on the BPRS remained significant. Moreover, no correlation between antipsychotic dosages (chlorpromazine equivalents) and serum total serine (r = −0.109, P = .59), L-serine (r = −0.111, P = .58), and D-serine (r = 0.270, P = .17) levels was observed in the medicated patients. In contrast, no significant correlation between total serine (total scores: r = 0.278, P = .32; positive symptoms scores: r = 0.465, P = .08; negative symptoms scores: r = 0.019, P = .95), L-serine (total scores: r = 0.281, P =.32; positive symptoms scores: r = 0.467, P = .08; negative symptoms scores: r = 0.022, P = .94), or D-serine (total scores: r = −0.051, P = .86; positive symptoms scores: r = 0.127, P =.66; negative symptoms scores: r = −0.189, P = .52) levels and the BPRS scores (total, positive, negative) was observed in the drug-naive patients. In addition, there was no significant difference regarding total serine (healthy controls: z =−0.536, P = .59; schizophrenic patients: z = −1.43, P = .15), L-serine (healthy controls: z = −0.506, P = .61; schizophrenic patients: z =−1.49, P = .14), or D-serine (healthy controls: z = −0.904, P = .37; schizophrenic patients: z = −0.521, P = .60) between smokers and nonsmokers among healthy controls and schizophrenic patients. Furthermore, no significant difference was observed among patients regarding total serine (H = 2.78, P =.60), L-serine (H = 2.77, P = .60), D-serine (H = 4.55, P = .34), or the ratio of D-serine to total serine (H = 4.47, P = .35) in terms of the disease subtypes (eg, catatonic, paranoid, residual, disorganized, and undifferentiated) determined by the DSM-IV criteria.