This study used a prospective, double-blind placebo-controlled study to determine the minimal dose of botulinum toxin (BTX) necessary to eliminate sternocleidomastoid (SCM) muscle activity in cervical dystonia (CD). Twelve patients with CD were randomized to receive 0, 20, 40, 60, 80, or 100 IU of BTX (diluted to a total volume of 1 cc) into the most clinically active SCM. Quantitative Eletromyographic (EMG) measures, clinical rating, and subjective patient assessments were obtained before and four weeks after BTX.A single injection of BTX into the SCM virtually eliminated muscle activity with all BTX doses administered (Manova, F[1,6] = 26.76, p = 0.002). Despite this marked reduction in SCM muscle activity, no significant clinical or subjective improvements were observed. Neck flexor weakness occurred in 59% and postinjection dysphagia occurred in 25%. Neither of these side effects were dose-related. We conclude that, with the injection techniques used in this study, even small doses of BTX can eliminate SCM muscle activity in CD and cause dysphagia and neck weakness. However, since CD is a disorder involving multiple muscles, more than one muscle must be injected with BTX for clinical efficacy. Similar studies of other cervical muscles may determine the minimum and the optimal effective doses of BTX for each muscle.