The abilities of Parkinson's disease (PD) patients, taking routine medication, and of control subjects, to discriminate bilateral differences in the static angular positions of the two elbow joints were studied during passive (subject relaxed) and active (subject contracting to hold position) conditions. On each trial, one of the subject's elbows served as the reference joint (angle 60Β°) and the other as the test joint (angular range, 54Β°to 69Β°, at 3Β°intervals). Subjects, with eyes closed, were required to discriminate the relative angles of the two elbows. In Experiments 1 (passive condition) and 2 (active condition), parkinsonians (n β«Ψβ¬ 12) gave significantly fewer correct responses, pooled across sides, than did controls (n β«Ψβ¬ 12), both in total scores across all angles and at individual test angles of 57Β°and 63Β°. In Experiment 3 (passive condition), derivation of conventional psychophysical variables indicated that both the difference limen (DL; threshold) and Weber ratio (WR; discriminatory sensitivity, independent of absolute stimulus values; same as DL/PSE) values of patients (n β«Ψβ¬ 6) were significantly larger than those of controls (n β«Ψβ¬ 6), in the absence of a significant difference between groups in the point of subjective equality (PSE). Our results provide clear evidence of a quantitative impairment of joint position sense in PD patients.