A longitudinal study over a mean of 32 months was conducted on 18 subjects with spastic diplegia, ranging in age from 4 to 14 years. Three-dimensional gait analyses were performed to compare the temporal and kinematic data across the two time intervals. The comparison revealed a deterioration of gait stability evidenced by increases in double support and decreases in single support with time and growth (p < 0.05). Kinematic analysis revealed a loss of excursion about the knee, ankle, and pelvis (p < 0.05). Additionally, passive range-of-motion analysis revealed a decrease in the popliteal angle over time (p < 0.05). In conclusion, this longitudinal investigation revealed that, in contrast to the gait of children with intact motor function, ambulatory ability tends to worsen over time in spastic cerebral palsy. Insight into the natural progression of gait function in cerebral palsy is essential when evaluating the change in motor status over time or the effects of an intervention in this population.