Sentinel node biopsy (SNB) has emerged as an accurate means of identifying nodal disease in patients with malignant melanoma. Superselection of pathological nodes has allowed improved pathological staging of disease. The aim of this study was to look at the impact of immunohistochemistry on pathological staging of sentinel nodes. The first 100 patients undergoing SNB for primary cutaneous malignant melanoma were included in this study. Sentinel node harvesting was performed with the aid of preoperative lymphoscintigraphy and the intraoperative use of both a gamma probe and blue dye. If the sentinel nodes contained tumour on either routine pathology or immunohistochemistry, patients were offered a therapeutic lymph node dissection (TLND). Patients underwent no other treatment to the primary lymph node basin if the sentinel node was free of metastases. In all, 95 patients had at least one node identified, and 25 were staged SNB positive and offered subsequent TLND. We found that 76% (19/25) of SNB positive patients were staged positive on routine pathology, and 24% (6/25) were staged with immunohistochemistry. Immunohistochemistry upstaged disease in 8% of patients (6/76). In all, 21 of the patients staged positive with SNB underwent TLND; 50% (8/16) of the patients staged sentinel node positive with routine pathology showed no further disease in the TLND, compared with 100% (5/5) of the patients staged sentinel node positive with immunohistochemistry only (P<0.05). Three patients have developed recurrence within the nodal basin following a negative SNB. The sensitivity of the procedure is currently 89% (25/28), with a mean follow-up of 24 months. Immunohistochemistry is an essential part of identifying micrometastasis in sentinel nodes, upstaging 8% of patients in our series. Patients with micrometastatic disease may well have a different prognosis from those with occult disease, and careful delineation of these patients is required to determine the prognostic influence of micrometastasis.