Aim: The aim of this study was to describe how the usefulness of the hemiplegic hand develops in children with unilateral cerebral palsy (CP) aged between 18 months and 8 years. Method: A prospective longitudinal study of 43 children (22 males, 21 females) with unilateral CP was conducted. Inclusion age was 18 months to 5 years 4 months (mean 2y 8mo [SD 1y 1mo]). Children were assessed with the Assisting Hand Assessment (AHA) 3 to 11 times per child over a mean period of 4 years 6 months. Two models were used for grouping children: by AHA score at 18 months and by Manual Ability Classification System (MACS) levels. Estimated average motor development curves were fitted with a nonlinear mixed-effects model. Results: Children with a high AHA score (high ability level) at 18 months reached a significantly higher ability level and at a higher progression rate than children with a low 18-month AHA score. Limits of development differed between the three MACS levels. Interpretation: Results indicate that the AHA score at 18 months can be used to discuss future development of affected hand use in bimanual tasks in children with unilateral CP.