Introduction: This study describes the results of internal mammary chain (IMC) biopsy, identifying factors that predict 'hot spots' and nodal metastases for patients in whom mapped IMC nodes were routinely dissected. Methods: The nodal basin and status of every axillary and IMC site identified by lymphoscintigraphy were examined. Binary logistic regression analysed the relationship of several patients and tumour factors with IMC hot spots and metastases. Results: Ninety of 490 patients (18.4%) had IMC sentinel lymph nodes (SLNs) identified by lymphatic mapping and dissected, and 20 of these (22.2%) were found to have metastases. Mapping to the IMC was most likely for women aged under 35 years (29.4%) (p = 0.117), women aged 35-44 (22.6%) (p = 0.034) or those with medial (23.7%) or central tumour location (22.2%) (p = 0.014; p = 0.062, respectively). Predictors of IMC positivity included age <35 years (p = 0.063), grade 3 histology (p = 0.018) and lymphatic vascular invasion (LVI) (p = 0.032). Although IMC positivity was more likely with positive axillary nodes, this trend was not significant. Conclusion: We identified several factors (age <35 years, tumour grade and LVI) that independently predict IMC SLN identification and positivity for patients with stage I or II breast cancer. Where IMC hot spots are not dissected, we predict IMC positivity of 50% or more for young women (<35 years) or women with high grade or LVI positive tumours, and these women may benefit from more intensive chemotherapy and radiotherapy to the IMC.