The present study aimed to examine the relationship between plasma concentration levels of risperidone and clinical effects in the treatment of delirium. We conducted a prospective, open-label, flexible-dose study of risperidone oral solution. Ten patients with delirium were assessed using Delirium Rating Scales. Plasma concentration levels of risperidone were measured 30 min after the first administration of a 0.5 mg dose. Two patients with high plasma levels had adverse effects and one patient with the lowest plasma level did not achieve remission; the remaining seven patients achieved remission without any adverse effects. A highly significant negative correlation was observed in these responders without adverse effects between the plasma levels and durations of treatment until remission (r=-0.861, P=0.0095). The plasma concentration level of risperidone at 30 min after the first 0.5 mg dose may be a favourable response predictor in the treatment of delirium. Further studies in larger samples are needed to verify this preliminary finding.