Controversy exists as to whether muscle weakness is present in Parkinson's disease (PD). Computerized literature searches identified clinical trials and reviews about muscular strength assessment in patients with Parkinson's disease, using the following databases: PubMed, Ovid MEDLINE, Ovid EMBASE, the Cochrane Database of Systematic Reviews, Cumulative Index to Nursing and Allied Health Literature, and Physiotherapy Evidence Database. Seventeen articles fulfilled all criteria for selection. These studies suggested that isokinetic muscle strength was decreased in patients with Parkinson's disease and that muscle weakness was not specifically related to tremor or rigidity. Bilateral asymmetrical muscle weakness was present in Parkinson's disease when presenting with clinical unilateral hemiparkinsonism. Recent studies using sensitive mechanical devices have provided evidence that muscle strength is reduced in patients with Parkinson's disease compared with age-matched controls. The specific cause of this weakness is not known. Questions under debate were whether this weakness was of central or peripheral origin and whether it was intrinsic to the disease or a secondary phenomenon.