ICU delirium occurs frequently and is associated with unfavorable outcome. A recent delirium prevention study, the REDUCE study, has assessed the efficacy of prophylactic use of haloperidol. In this subset of patients, included in the REDUCE study, we found a roughly linear dose–concentration relation of haloperidol trough concentrations. The high serum level in two patients may reflect poor metabolism, possibly caused by genetic polymorphism of CYP2D6 that is involved in haloperidol metabolism. Overall, we found no relation between serum trough level and the occurrence of delirium or the number of delirium-free-days.The haloperidol serum levels were lower in the two treatment groups than those reported in a treatment set-ting for patients with schizophrenia and other psychotic disorders. It is unknown whether these low serum levels have any relation to the absence of effect of prophylactic haloperidol in the REDUCE study. Further study should focus on serum levels of higher doses of haloperidol and the relation to clinical effects.

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