The National Surgical Adjuvant Breast and Bowel Project B-32 trial is examining whether patients with initially negative sentinel lymph nodes (SLNs) who have occult metastases detected on deeper levels and cytokeratin immunohistochemistry stains are at risk for regional or distant metastases. The experimental B-32 protocol was designed to detect metastases larger than 1.0 mm by examining sections approximately 0.5 and 1.0 mm deeper into the paraffin blocks (2 levels; wide spacing). This pilot quality assurance study compares detection rates to a comprehensive protocol designed to detect metastases larger than 0.2 mm (multilevel; narrow spacing). All SLNs were sectioned grossly at close to 2.0 mm and all sections embedded in paraffin blocks. For clinical treatment, a single hematoxylin and eosin section was examined from each block. For 54 cases with 1 to 5 SLNs and all SLNs negative, additional cytokeratin immunohistochemistry sections were evaluated every 0.18 mm through the block until no tissue remained. Twenty of 176 (11.4%) blocks harbored occult metastases; the B-32 protocol detected metastases in 11 blocks (6.3%) and 9 additional blocks (5.1%) with metastases were detected on sections that would not have been evaluated (P=0.002; correlated proportions). Median number of levels examined per block on the comprehensive protocol was 11 (range: 3 to 26); the B-32 protocol was fixed at 2 levels (median 2; range: 1 to 2). Median thickness of node sections in the block was 2.1 mm (range: 0.7 to 4.8 mm) and the modal thickness was 2.3 mm. Although more comprehensive sectioning of SLNs detects additional micrometastases, the data suggest diminishing returns and reduced cost effectiveness for the comprehensive strategy.