Summary:Recent preliminary findings by Williams et al. of increased platelet monoamine oxidase (MAO) activity after prednisone administration and anecdotal reports of associated behavior changes were replicated in a planned study. Children in a standard chemotherapy program for acute lymphocytic leukemia (ALL) that requires periodic steroid administration were studied. Systematic assessments of platelet MAO activity, mood, behavior, and cognitive–motor functioning were conducted prior to, during, and after prednisone treatment. Increased platelet MAO activity correlated with deterioration in mood and behavior (resembling atypical depression), but had no effect on cognitive–motor functions.