Background: Botulinum toxin type A (BT-A) is increasingly being used by dermatologists for correction of frown lines. Because objective measurements of clinical results appear to be difficult, several different treatment protocols have been issued purely empirically or on the basis of subjective ratings. Objective: Our purpose was to establish objective parameters to measure the efficacy of BT-A for correction of hyperkinetic facial lines. Methods: Thirty consecutive patients received BT-A injections for correction of facial expression lines. For each patient a full range of facial expressions was recorded by means of a digital imaging system that allowed identical positioning and illumination before and after treatment. Computer-assisted measurements of brow mobility were used to measure muscular paralysis. Results: Reproducibility of serial photographs by means of a digital overlay technique was confirmed by 4 independent observers. Upward mobility of brows was decreased to 35% at 2 weeks and 71% at 12 weeks after treatment. In contrast, inward mobility (frowning) was decreased to 7% at 2 weeks and 57% at 12 weeks. Brow-to-brow distance in repose increased with treatment by 13% and displayed a negative correlation with age. Conclusion: The effects of BT-A on upper face muscular activity can reproducibly be measured by digital image analysis; this is a valuable tool for clinical documentation and evaluation of treatment efficacy. Onset and offset of the effects of BT-A display a longer time course than previously assumed. Tissue qualities such as elasticity contribute measurably to smoothing facial expression lines after BT-A treatment and correlate inversely with age. (J Am Acad Dermatol 2001;45:508-14.)