We conducted an open-label study to determine the utility of treating severe hand tremors with intramuscular injections of botulinum toxin (BTX) in forearm and arm muscles in 26 patients, 12 with Parkinson's disease (PD) and 14 with essential tremor (ET). The effect after 6 weeks for each patient was evaluated using two clinical rating scales, subjective evaluations of functional improvement and global disability, measures of weakness, and computer-assisted quantitative assessments of tremor. Although none of the clinical scores averaged > 3/4 point change, statistical significance was found on comparison of pre- and postinjection scores in the Webster Tremor and Global Disability Scales in the ET patients. Similarly, although average tremor amplitudes decreased by no more than 25% by quantitative analysis, amplitude decrease significantly correlated with patient subjective assessment in ET. In only two of 12 PD (17%) and three of 14 ET patients (21%) were major quantitative changes in tremor amplitude (> 50% reduction) found after BTX injections. Nevertheless, 10 patients (38%; five PD and five ET) reported moderate to marked subjective improvement in functional benefit after BTX. These findings suggest that although there were no major changes in clinical ratings or objective measurements, BTX injections may subjectively improve tremor in some patients, particularly those with ET.