Background: There has been little examination of force production of the upper limb in people with Parkinson's disease (PD), despite its impact on activities of daily living and clear evidence that force production is significantly reduced in lower limb muscle groups. The aim of this study was to determine the force production of the major muscle groups of the upper limb in people with PD during the “on” phase after medication, compared with aged-matched neurologically-normal controls. Method: A cross-sectional study was carried out. Participants: Thirty people with mild PD (Hoehn Yahr mean 1.1) and 24 age-matched neurologically-normal controls. Outcome measures: Maximum isometric force production of the shoulder flexors, extensors, abductors, adductors, internal rotators and external rotators, elbow flexors and extensors, wrist flexors and extensors and hand grip using dynamometry. Results: There was a significant impairment in force production in all upper limb muscle groups, compared with control participants, except in the wrist flexors. On average the deficit in force production was 22%, despite people with PD having mild disease, being physically active and being measured during the “on” phase of medication. The most severely affected muscle groups were the upper limb extensors. Conclusion: People with PD have a significant deficit in force production of the upper limb muscle groups compared with age-matched neurologically normal controls. Clinical Implications: Regular assessment of strength of the upper limb should be considered by clinicians and strengthening interventions could be implemented if a deficit is identified.