Mirels’ rating system is commonly used to predict risk of fracture in patients with metastatic bone lesions to long bones, but it has not been independently validated for use in humeral bone lesions. We asked whether this system was a valid and reproducible instrument for predicting impending pathologic fractures in the humerus. We presented 17 case histories and plain radiographs of 16 patients with humeral metastases through a web-based survey to 39 physicians with varying training and experience. Participants scored each case using Mirels’ criteria and provided a fracture prediction, which was compared with actual outcome in the subset of 12 patients with three fractures not treated prophylactically. Using Mirels’ definition of impending pathologic fracture (nine points or greater), the sensitivity and specificity for determining the likelihood of pathologic humeral fracture were 14.5% and 82.9%, respectively. When we used seven or more points as the definition of impending pathologic humeral fracture, sensitivity improved to 81% but specificity was reduced to 32%. Kappa analysis suggested moderate reproducibility across groups for prediction of pathologic fracture. The Mirels rating system for humeral lesions is reproducible and valid, but low specificity at acceptable sensitivity levels as reported remains a problem as for femoral lesions.