In order to quantitatively assess the motor performance characteristics of 14 patients with congenital myotonia, the reaction time, speed of movement, synergy of different muscle groups and accuracy were measured. The Human Performance Measurement/Basic Elements of Performance device was used for recordings. Warned simple and choice reaction times (SRT, CRT) were significantly longer in the myotonic patients than in the controls. SRTs, consisting of one constant visual stimulus followed by a single movement response of the upper extremities (patients vs. controls) were 218 ± 48 ms (mean ± SD) and 172 ± 12 (p = 0.0038). In the lower extremities the corresponding results were 293 ± 46 and 239 ± 24 (p = 0.0018). 1-CRTs, consisting of the upper extremities response to one randomized light signal (patients vs. controls) were 265 ± 45 and 218 ± 26 (p = 0.0069), and those of the lower extremities 337 ± 73 and 279 ± 39 (p = 0.0107), respectively. 2-CRTs, consisting of two possible visual stimuli in randomized order followed by a movement response of the upper extremities (patients vs. controls), were 308 ±54 and 249 ± 33 (p = 0.0018), and those of the lower extremities 387 ± 53 and 323 ± 46 (p = 0.0028), respectively. We did not find any significant difference between the patient and the control groups in speed of movement, synergy of different muscle groups or accuracy. Nor was any significant correlation between the motor performance disability and the disease se