The tremor present in a small number of patients who present with features of both Parkinson's disease (PD) and essential tremor (ET) is ill-defined. We therefore studied 8 such patients with 'atypical' tremor and compared them clinically to 11 PD and 10 ET patients in a double-blind, cross-over, placebo-controlled trial. Tremor was assessed via accelerometry and surface EMG. Tremor frequency and the acute tremorlytic response to propranolol were not useful in discriminating between groups. Over 80% of atypical and PD patients showed alternating EMG, whereas 90% of ET patients had synchronous EMG activity. Tremor amplitude improved by > 90% in 38% of atypical, 27% of PD and none of the ET patients after single-dose L-dopa/benserazide (300/75 mg), a response which was significantly different between the ET and PD groups. Failure to differentiate electrophysiologically and pharmacologically between the atypical and other groups probably relates to clinical heterogeneity in the atypical group.