Schizophrenic patients taking haloperidol prophylactically were screened for a haloperidol monitoring project at four medical centers. Fifty-five patients participated. Most patients had doses and plasma levels below those found in acute treatment studies; however, a sizeable minority had doses and levels that were greater than was probably needed for prophylaxis. Six percent of patients appeared to be rapid metabolizers. A regression formula was calculated for prediction of haloperidol level from daily dose. There were no apparent differences in dosing practices between two university- affiliated medical centers and two nonaffiliated centers. More research is needed to establish optimal prophylactic haloperidol levels. However, haloperidol monitoring can provide some assistance for choice of dose.