BACKGROUND: Our aim was to make a comparative analysis of radiological and clinical outcomes of using either one or two interlocking distal screws on a Polarus intramedullary nail for the internal fixation of humeral shaft fractures. METHODS: From January 2008 to March 2014, we enrolled 26 patients with humeral shaft fractures who were operated on using intramedullary nails. The patients were divided into 2 groups according to how many interlocking distal screws were used to lock the Polarus nail: in group 1, a single interlocking distal screw was used in 12 patients; and in group 2, double interlocking distal screws, in 14 patients. We compared the degree of recovery of the displaced fracture fragments between the two groups. To compare the nonunion and shoulder function, we assessed each patient's modified American Shoulder and Elbow Surgerns (ASES) score. RESULTS: We found that 10 of 12 fractures achieved union in group 1, and 13 of 14 fractures, in group 2. We did not find a meaningful difference in the time to bone union between the two groups. The percentage of recovery of displaced fracture fragments until union was 66.9% for group 1 and 59.41% for group 2. At the final follow-up, we found that the scores for shoulder joint modified ASES was 78.7 for group 1 and 80.7 for group 2. CONCLUSIONS: Our results show that if locked appropriately, even a single screw on a Polarus nail can provide satisfactory radiological union and improved clinical outcome after intramedullary nailing of humeral shaft fractures.