Objective: To investigate relationships between amount of use of the more affected upper extremity and functional motor and communication performance classification systems. Material and methods: The study comprised 95 children with congenital hemiplegic cerebral palsy (CP) aged 6–15 years (52 males, 43 females; mean age 9.53, SD 3.1) and their parents/caregivers. The amount of use of the more affected upper extremity was assessed using Pediatric Motor Activity Log-Revised-How Often subscale (PMAL-R HO). Functional levels of the enrolled children were defined by the parents/caregivers using Manual Ability Classification System (MACS), Gross Motor Function Classification System-Expanded and Revised (GMFCS-E&R), and Communication Function Classification System (CFCS). Results: A strong and negative correlation was found between PMAL-R HO subscale score and MACS (r = −0.819), suggesting that children with lower MACS levels are more likely to use their more affected upper extremity spontaneously. Additionally, negative and moderate associations between PMAL-R HO subscale score and GMFCS and CFCS were revealed (r1 = −0.549 and r2 = −0.567). Conclusion: The amount of use of the more affected upper extremity is more sensitive to MACS than GMFCS-E&R and CFCS. Children with a given MACS level had a wide range of PMAL-R HO subscale score. In addition to MACS, a score on the PMAL-R HO subscale related to the more affected upper extremity should be included as an inclusion criterion in clinical trials to avoid misleading effects of intervention approaches aimed at improving the amount of use of the more affected upper extremity in children with congenital hemiplegic CP.