The aim of this study was to investigate the reliability and sensitivity of the Wartenberg pendulum test for quantification of muscle tone in young children with spastic diplegia undergoing selective dorsal rhizotomy (SDR). Fourteen non-disabled children (mean age of 5.5 years, age range 2.3 to 8.8 years, one female and one male in each year) were tested twice. Twenty children with spastic diplegia (12 males, eight females; mean age of 4.3 years, age range 2.5 to 6.3 years) consecutively selected for SDR, were assessed before and 6 months after SDR. Parameters of the pendulum test: R2, R1, maximal velocity, and swing time were correlated with clinical assessments for spasticity (modified Ashworth scale, quadriceps reflex) and measurements of gross motor function: the Gross Motor Function Classification System and the Gross Motor Function Measure. The Wartenberg pendulum test was found to be an objective and sensitive method for quantifying spasticity in knee extensor muscles in children as young as 2.5 years old. The method was responsive to changes after SDR. The only correlation with clinical measurements of spasticity was between the R2 ratio and the quadriceps reflex. Swing time was the most reliable and sensitive variable; it showed a weak correlation with measurements for gross motor function.