Primary shoulder impingement syndrome is a common shoulder problem which, if treated ineffectively, can lead to more serious pathology and expensive treatment. This study examined whether subjects receiving joint mobilization and comprehensive treatment (hot packs, active range of motion, physiologic stretching, muscle strengthening, soft tissue mobilization, and patient education) would have improved pain, mobility, and function compared with similar patients receiving comprehensive treatment alone. Subjects were eight men and six women (mean age = 52.9 years) with primary shoulder impingement syndrome (superolateral shoulder pain, decreased active humeral elevation, limited overhead function). Following random assignment to experimental (N = 7) and control groups (N = 7), three blinded evaluators tested 24-hour pain (visual analog scale), pain with subacromial compression test (visual analog scale), active range of motion (goniometry), and function (reaching forward, behind the head, and across the body in an overhead position) before and after nine treatments. One-tailed analyses of covariance (baseline values as covariates) showed that the experimental group had less 24-hour pain and pain with subacromial compression test but no differences in range of motion and function (Mann-Whitney U) compared with controls. The experimental group improved on all variables, while the control group improved only on mobility and function (one-tailed, paired t tests; Wilcoxon matched pairs). Age, side of dominance, duration of symptoms, treatment attendance, exercise quality, and adherence had no effect on the outcomes. Results may be affected by inadequate sample size, minimal capsular tightness, insensitive functional scale, nonspecific motion measurements, position at which mobilization treatment was given, or a strong effect of comprehensive treatment. Mobilization decreased 24-hour pain and pain with subacromial compression test in patients with primary shoulder impingement syndrome, but larger replication studies are needed to assess more clearly mobilization's influence on motion and function.