Plasma levels of haloperidol were monitored in children and teenagers suffering from psychotic episodes and/or abnormal movements (tics and Gilles de la Tourette''s syndrome). Steady-state concentrations of haloperidol ranged from 0.7-19 ng/ml without any apparent relationship with the administered dose (15-285 .mu.g/kg per day) and a 15-fold variability was observed for the same daily dosage. On the contrary, a significant (P < 0.02) relationship was found between the age of the patients and the plasma concentrations to dose ratios, lower values being present in younger patients. Side effects too appeared to be related to plasma levels, with a significant increase (P < 0.01) in incidence for concentrations over 6 ng/ml. In most of the cases suffering from tics and Gilles de la Tourette''s syndrome, a positive response was associated with plasma levels of 1-4 ng/ml, while no relationship could be established for the psychotic group. The relevance of monitoring plasma drug levels when prescribing haloperidol in pediatrics is discussed.