Background: Mini-open reduction and percutaneous fixation of proximal humeral fractures historically results in good outcomes and a low prevalence of osteonecrosis reported with short-term follow-up. The purpose of this study was to determine the midterm results of our multicenter case series of proximal humeral fractures treated with percutaneous fixation. Methods: Between 1999 and 2006, thirty-nine patients were treated with percutaneous reduction and fixation for proximal humeral fractures at three tertiary shoulder referral centers. Twenty-seven of these patients were available for intermediate follow-up at a minimum of three years (mean, eighty-four months; range, thirty-seven to 128 months) after surgery; the follow-up examination included use of subjective outcome measures and radiographic analysis to identify osteonecrosis and posttraumatic osteoarthritis on radiographs. Results: Osteonecrosis was detected in seven (26%) of the total group of twenty-seven patients at a mean of fifty months (range, eleven to 101 months) after the date of percutaneous fixation. Osteonecrosis was observed in five (50%) of the ten patients who had four-part fractures, two (17%) of the twelve patients who had three-part fractures, and none (0%) of the five patients who had two-part fractures. Posttraumatic osteoarthritis, including osteonecrosis, was present on radiographs in ten (37%) of the total group of twenty-seven patients. Posttraumatic osteoarthritis was observed in six (60%) of the ten patients who had four-part fractures, four (33%) of the twelve patients who had three-part fractures, and none (0%) of the five patients who had two-part fractures. Conclusions: Intermediate follow-up of patients with percutaneously treated proximal humeral fractures demonstrates an increased prevalence of osteonecrosis and posttraumatic osteoarthritis over time, with some patients with these complications presenting as late as eight years postoperatively. Development of osteonecrosis did not have a universally negative impact on subjective outcome scores. Level of evidence: Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.