Abstract Background The aim of this study was to evaluate the influence of patient and adenoma characteristics on 99mTc–methoxy isobutyl isonitrile (MIBI) scan performance in individuals diagnosed with primary hyperparathyroidism (PHP). Methods Records of patients undergoing parathyroidectomy for PHP over 6 years at a single center were reviewed. Results The overall true-positive (TP) rate for 99mTc-MIBI scans was 56%. Adenomas sized 1.9 to 3.5 cm were more likely to have TP scans than 0.3-cm to 1.8-cm adenomas (74% vs 40%, P < .001). Preoperative ionized calcium levels between 1.49 and 1.72 mmol/L were more likely to have TPs than levels between 1.27 and 1.48 mmol/L (65% vs 47%, P < .05). No single class of medication was shown to significantly effect TP rates. A decrease in TP rate was observed for larger adenomas in patients on ≥1 medication (74% vs 65%, P = .05). Conclusions In PHP, 99mTc-MIBI scan positivity is most related to adenoma size and preoperative ionized calcium level.