['Severe generalized spastic movement disorders of various aetiologies often involve the jaw muscles and lead to a spastic trismus with masseter muscle hypertonia. We report a placebo-controlled randomized study on patients with spastic trismus.', 'Eleven patients with masseter hypertonia because of stroke, hypoxic encephalopathy or traumatic brain injury were allocated to either botulinum toxin serotype B (BoNT/B) injections into the masseter muscles or placebo treatment. The dental gap, the amount of saliva, salivation scales, and a clinical goal attainment were evaluated.', 'Three weeks after injection the BoNT/B group showed a significantly increased mouth opening compared with placebo treatment (P < 0.05). In addition to the muscle paralysing effect, a goal attainment scale demonstrated a clinical benefit for the BoNT/B group (P < 0.01).'] Does botulinum toxin B increase mouth opening in patients with spastic trismus?