OBJECTIVE Tc-99m Hydroxymethylene diphosphonate (HMDP) bone scintigraphy is commonly used to diagnose bone disorders. We aimed to quantify and characterize the occurrence of radiopharmaceutical extravasation in bone scintigraphy, using Tc-99m HMDP, as well as to compare the visual classification of the events with an independent analysis using image processing software. METHODS We conducted a cross-sectional study, using data from a total of 400 (9.1%) exams, randomly selected from all the procedures performed in 2018 in the Portuguese Institute of Oncology of Porto, Portugal. Prevalence estimate and the corresponding 95% confidence interval (CI) was computed for the presence of extravasation. Odds ratios and 95% CI were computed to quantify the association between demographic and clinical characteristics, and the occurrence of extravasation. RESULTS The prevalence of Tc-99m HMDP extravasation was 26.5% (95% CI: 22.4-31.0). Those from an inpatient setting had almost seven-fold higher odds of extravasation than those from an outpatient setting. When the wrist was used for administration, there was three times more odds of extravasation when compared to the use of hand. There were statistically significant differences in the median scores of extravasations severity obtained from image processing software according to the different grades attributed by visual appreciation ( P  < 0.001). CONCLUSIONS Tc-99m HMDP extravasation occurred in one out of four patients, being more frequent among those from an inpatient setting and when the wrist was used for administration. Visual appreciation of the extravasation seems to be acceptable to classify its severity.