To investigate the effects of secondary motor tasks of three levels of difficulty on the spatial and temporal parameters of gait in subjects with Parkinson disease (PD) compared with control subjects.A two-group repeated measures design using a sample of convenience. Subjects performed 10-meter gait trials walking (1) freely, (2) carrying a tray, and (3) carrying a tray with four plastic glasses.Subjects were tested in the gait laboratory at Kingston Centre, Victoria, Australia.Twelve subjects with PD and 12 control subjects matched for age, height, and sex were recruited from the Movement Disorders Clinic at Kingston Centre.Gait speed, stride length, cadence, and the proportion of the walking cycle spent in double limb support were measured in moderately disabled subjects with PD and control subjects.For all of the walking conditions, subjects with PD walked more slowly (F(1,22) = 16.54, p = .001, partial nu2 = .429) and with shorter steps (F(1,22) = 15.07, p = .001, partial nu2 = .406) than control subjects. In addition there were significant group by condition interaction effects for gait speed (F(2, 44) = 4.42, p = .018, partial nu2 = .167) and stride length (F(2, 44) = 4.95, p = .012, partial nu2 = .184). There was little deterioration in gait when subjects in either group carried a tray while walking compared with free walking; however, when required to carry four plastic glasses on the tray while walking, subjects in the PD group showed marked deterioration in gait speed (t(11) = 3.19, p = .009, alpha = .025) and stride length (t(11) = 3.82, p = .003, alpha = .025). Performance in the control subjects changed only marginally across the conditions.Subjects with moderate disability in PD experience considerable difficulty when they are required to walk while attending to a complex visuomotor task involving the upper limbs.