Introduction: Locking plate osteosynthesis of proximal humeral fractures are widely recommended and used, even in complex intraarticular fracture patterns such as AO/OTA Type C fractures. We systematically reviewed clinical studies assessing the benefits and harms of osteosynthesis with angle stable plates in AO/OTA Type C fractures of the proximal humerus. Methods: We conducted an iterative search in PubMed, Embase, Cochrane Library, Web of Science, Cinahl, and PEDro in all languages from 1999 to November 2010. Eligible studies should study the outcome for Type C fractures after primary osteosynthesis with locking plate within two weeks of injury, and a follow-up period of six months or more. Patients should be evaluated with the Constant-Murley Score (CS). Two observers extracted data independently. Results: Twelve studies and 282 Type C fractures were included. Results were categorised according to study type and synthesised qualitatively. No randomised clinical trials were identified. Two comparative, observational studies reported a mean CS of 71 (relative to contralateral shoulder) and 75 (non-adjusted Constant Score) for Type C fractures. For all studies mean non-adjusted CS ranged from 53 to 75. Mean age- and sex-adjusted CS ranged from 60 to 88. Mean CS relative to the contralateral shoulder ranged from 71 to 85. The most common complications were avascular necrosis (range, 4-33%), screw perforations (range, 5-20%), loss of fixation (range, 3-16%), impingement (range, 7-11%) and infections range 4-19%. Reoperation rate ranged from 6 to 44%. Conclusions: Insufficient study designs and unclear reporting preclude safe treatment recommendations. Complication and reoperation rates were unexpected high. Based on the studies included we cannot routinely recommend the use of locking plates in AO/OTA Type C fractures.