The purpose of this study was to determine whether botulinum toxin A (BTX-A) was efficacious for the treatment of chronic moderate to severe jaw muscle pain in females. This was a randomized double-blind, placebo-controlled crossover trial of BTX-A. Twenty five units injected into each temporalis muscle and 50 U injected into each masseter muscle using three sites per muscle with 0.2 cm(3) per site. Data were collected at baseline, 8, 16, 24 weeks, with crossover occurring at 16 weeks. Primary outcome variables were pain intensity and unpleasantness, measured by horizontal visual analog scale (VAS). Secondary outcome variables were maximum interincisal opening without and irrespective of pain, muscle palpation tenderness (12 points), and four general questions. Fifteen female patients were enrolled (18-45 years), but only ten completed the trial. Of those who finished, no statistically significant difference was found in pain intensity (P=0.10), unpleasantness (P=0.40), palpation muscle tenderness (P=0.91), or the three general questions (P=0.64, P=0.66, P=0.67). Statistical significance was achieved for maximum opening without pain (P=0.02) and irrespective of pain (P=0.005) with the BTX-A arm having a relative decreased opening. No statistically significant difference was observed in any outcome measures except maximum opening, which showed BTX-A patient opening less wide than placebo. The results do not support the use of BTX-A in the treatment of moderate to severe jaw muscle pain in this patient population.