Background: The ability to sit independently is fundamental for function but delayed in infants with cerebral palsy (CP). Studies of intervention directed specifically toward sitting in infants with CP have not been reported. Objective: Our purpose was to compare two interventions for improving sitting postural control in infants with CP. Design: For this randomized longitudinal study, infants under 2 years old and at risk for CP were recruited for intervention directed toward sitting independence. Setting: The intervention was conducted at home or at an outpatient facility. Patients: Fifteen typically developing infants (5 months old, SD .5 months at entry) were followed longitudinally as a comparison for postural variables. Thirty-five infants with delays in achieving sitting were recruited. Infants with delays were randomly assigned to a home program (1x/week for 8 weeks; mean age=15.5 months, SD=7 months), or a perceptual-motor intervention (2x/week for 8 weeks; mean age=14.3 months, SD=3 months). Measurements: The primary outcome measure was Center of Pressure (COP) data, from which linear and nonlinear variables were extracted. The Gross Motor Function Measure (GMFM) sitting subsection was our clinical outcome measure. Results: There was a main effect of time in the GMFM sitting subscale and in two of the COP variables. Interaction of group by time factors indicated significant differences between intervention groups on two COP measures, in favor of the group with perceptual-motor intervention. Limitations: The small number of infants limits the ability to generalize the findings. Conclusions: Although both groups made progress in the GMFM, the COP measures indicated an advantage for the group with perceptual-motor intervention. The COP measures appear sensitive for assessment of infant posture control and quantifying intervention response.