Activity of the biceps brachii muscles and movements of the elbows were studied during associated reactions in hemiparetic patients and in healthy volunteers. Onset time and increase in electromyographic (EMG) activity during associated reactions and onset of elbow flexion and its maximal magnitude were measured. Testing was performed while standing with a footswitch attached to the sole of the nonparetic foot in patients and to the sole of the right foot in controls. Lifting of that foot generated a trigger signal that served to time the dependent variables. Bilateral EMG activity associated with one foot stance appeared in the two upper extremities in both patients and controls. Elbow flexion occurred in the majority of patients bilaterally, whereas in controls it frequently took place on one side only. There was a significant difference between patients and controls in onset of EMG activity and elbow flexion. This difference indicates an earlier preparatory activity to one foot stance in the upper limbs of patients than in healthy controls. The greatest excursion into flexion was measured in the paretic upper extremity of patients; it significantly exceeded both the flexion angle measured in controls and increase in flexion angle on the nonparetic side. Further understanding of the nature of associated reactions seems to be required for their adequate treatment by physical procedures.