Background: Repetitive shoulder movements during competitive training may cause changes in the strength of periarticular shoulder structures in preadolescent swimmers. Objective: To prospectively determine the effects of training on shoulder periarticular structures and muscle strength in preadolescent swimmers. Design: Prospective cohort study. Setting: Community-based natatorium. Participants: Twenty-four preadolescent swimmers aged 10–12 years. Interventions: Not applicable. Main Outcome Measures: Measurements were repeated in three periods as preseason, midseason, and postseason. Ultrasonographic measurements (supraspinatus tendon thickness, humeral head cartilage thickness, deltoid muscle thickness, and acromiohumeral distance) were performed using a portable device and a linear probe. Shoulder (flexion, extension, abduction, internal and external rotation) and back (serratus anterior, lower, and middle trapezius) isometric muscle strength were measured with a handheld dynamometer. Results: Supraspinatus tendon thickness and acromiohumeral distance were similar in all periods (all p >.05); however, deltoid muscle and humeral head cartilage thicknesses increased throughout the season (p =.002, p =.008, respectively). Likewise, whereas shoulder muscle strength increased (all p <.05), back muscle strength was similar in all periods (all p >.05). Conclusions: In preadolescent swimmers, acromiohumeral distance and supraspinatus tendon thickness seem to not change; but humeral head cartilage and deltoid muscle thicknesses as well as shoulder muscle strength increase throughout the season.