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Increasing evidence indicates that enhanced peripheral carbonyl stress markers exist in subtype of schizophrenia, although it may not be the primary cause. This study aimed to investigate whether plasma concentrations of methylglyoxal, 3-deoxy-glucosone, and glyoxal, which are reactive intermediates of protein metabolism in carbonyl stress, are changed in patients with schizophrenia and can function as potential biomarkers for schizophrenia with enhanced carbonyl stress. Plasma concentrations of these di-carbonyls were simultaneously estimated in 40 patients with schizophrenia and 40 healthy controls. As a result, no statistically significant differences were observed in mean plasma concentrations of three di-carbonyls between patients and controls. However, a remarkable increase in methylglyoxal concentrations was observed in four patients but not in controls. This increase was not found with regard to 3-deoxyglucosone and glyoxal both of patients and controls. Our correlation analysis showed that both the plasma methylglyoxal and glyoxal concentrations were significantly correlated with 3-deoxyglucosone concentrations in 40 patients and 40 controls. However, the plasma methylglyoxal concentrations did not show any significant correlation with the glyoxal concentrations in the patients or the controls. In four patients with extremely high methylglyoxal levels, the plasma methylglyoxal and glyoxal concentrations were not correlated to the 3-deoxyglucosone concentrations. Methylglyoxal is a physiological substrate of the glyoxalase system, and the accelerated accumulation of this compound lowers the glyoxalase I activity. These results suggested that this increase in four patients with high methylglyoxal levels may indicate the presence of a subtype of chronic schizophrenia that is associated with enhanced carbonyl stress.