The aim of this single case study was to determine the effectiveness of a modified version of constraint-induced movement therapy (mCIMT) on a child less than one year of age with a diagnosis of hemiplegic cerebral palsy. A single-subject ABAB design with a 6-month follow-up evaluation used repeated measures of gross and fine motor skills to determine changes at each phase of the study. Measures included the Peabody Developmental Motor Scale-2 (PDMS-2), the Gross Motor Fine Motor Measure-88 (GMFM-88) and videotape analysis of specific motor skills typically seen in children less than one year of age. The child in this study participated in a conventional occupational and physical therapy for 2 hours a week during the 2 baseline phases, A1 and A2, and mCIMT during the 2 intervention phases, B1 and B2. The mCIMT involved constraint of the non-affected limb for 1-hour a day for 30 consecutive days as the child was engaged in developmentally appropriate, task specific activities implemented by therapists and parents. Following participation in this mCIMT, the child demonstrated clinical improvements in both gross and fine motor skills as measured by standardized assessments and videotape analysis of motor behaviors. He was completing developmental motor tasks at his chronological age despite motor deficits resulting from a right-sided hemiparesis. The results of this study supports the use of mCIMT for children less than one year of age and could shift the focus of future research studies to determining the age in which to implement mCIMT before patterns of learned non-use begin to affect the normal development of skilled motor movements in children with hemiplegic CP.