Time-frequency analysis methods were applied to surface EMG records of patients with tremor. Variation in tremor frequency over time and between muscles was measured in subjects with Parkinson's disease (n = 20), essential tremor (n = 8) and psychogenic tremor (n = 7). The effect of externally paced voluntary contractions on tremor frequency was also characterized. Psychogenic tremor involved fewer limbs and fewer limb segments than Parkinson's disease rest tremor and essential tremor, and its frequency was less consistent. In all subject groups, muscles within a single extremity generally had identical instantaneous frequencies. Frequency dissociation, used here to describe a modal contemporaneous frequency difference of more than 0.1 Hz between two extremities, was demonstrated for symptomatic tremors in 17 subjects with Parkinson's disease, in four subjects with essential tremor and in none of the subjects with psychogenic tremor. Dissociation between tapping and tremor limbs was demonstrated in an additional two subjects with Parkinson's disease and in all four remaining essential tremor subjects but in none of the psychogenic tremor subjects. Tremor maintained a different frequency from the tapping limb in Parkinson's disease and essential tremor, and its frequency in many cases shifted by at least 0.3 Hz compared with the non-tapping condition. For example, arm and leg tremors at 5.2 and 3.8 Hz, respectively, shifted to a common frequency of 4.6 Hz in one Parkinson's disease patient while using the contralateral arm to perform a tapping movement in time with a metronome at 2 Hz. These observations suggest the existence of distinct oscillator systems projecting to each tremoring limb, which can be linked to a variable degree, and which can be modulated by voluntary activation of another limb. Psychogenic tremor was not maintained while tapping with the contralateral arm: tremor either dissipated or shifted to the metronome's frequency. The latter response was also seen in normal volunteers mimicking tremor, but not in Parkinson's disease or essential tremor. We suggest that maintenance of phasic contraction in psychogenic tremor is not due to intrinsic instability of the motor system and that muscle activation in involved limbs may instead be synchronized to a common oscillator. As in voluntary movements, only a single rhythm may be easily followed at a time. Coexistence of muscle groups phasically contracting at consistently different instantaneous frequencies is evidence against a psychogenic aetiology of tremor.