AIM: To describe the development of the Mini-Assisting Hand Assessment (Mini-AHA) for children with signs of unilateral cerebral palsy (CP) aged 8 to 18 months, and evaluate aspects of content and internal scale validity. METHOD: The ability of the video-recorded Mini-AHA play session to provoke bimanual performance in children with unilateral CP and typical development was evaluated. Original AHA test items were examined for their suitability for younger children and possible new items were generated. Data from 108 assessments of children with unilateral CP (86 children, 53 males, 33 females; mean age 13 mo, SD 3 mo, range 8-18 mo) were entered into a Rasch measurement model analysis to evaluate internal scale validity. A Spearman's correlation analysis explored the relationship between age and ability measures for children with unilateral CP. The frequency of maximum scores in 40 children with typical development (22 males, 18 females; mean age 12 mo, SD 3 mo) was examined. RESULTS: The Mini-AHA play session provoked bimanual responses in typically developing children 99% of the time. Person and item fit criteria established 20 items for the scale. The resultant unidimensional scale also demonstrated excellent discriminative features through high separation reliability. The item calibration values covered the range of person ability measures well. Age was not related to the ability measures for children with unilateral CP (rs =0.178). All children with typical development achieved maximum scores. INTERPRETATION: Accumulated evidence shows that the Mini-AHA validly measures use of the affected hand during bimanual performance for children with unilateral CP aged 8 to 18 months. The Mini-AHA has the potential to be a useful assessment to evaluate functional hand use and the effects of intervention in an age group when potential for change is high.