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Age and body mass index (BMI) have been shown to correlate with an increased incidence of failure in identifying a sentinel lymph node (SLN). Mapping senior, overweight adults is common; therefore, the relationship of patient age and BMI on SLN biopsy success is essential. This study examines the mapping failures as they relate to age and BMI. From April 1994 to May 1999, patients underwent an injection of radiocolloid (450 mci) and blue dye (5 cc) prior to SLN biopsy. SLN biopsy failure was defined as lymph nodes being unidentifiable by blue dye or having an in vivo node radiocolloid count of less than 3:1 over background count. BMI was measured as (weight in pounds)(703)/(height in inches)(2); 1,356 patients were attempted for SLN mapping, and 54 failed (3.98%). The radioactive node count was inversely proportional to age ( p < 0.0001). The radioactive node count decreased by a mean of 34 counts per node with each additional year ( p < 0.001). The estimated odds ratios for success were 0.945 for age and 0.946 for BMI. Therefore, every increase of 1 year of age or one unit of BMI decreased the odds of success by approximately 5%. The mean BMI was 29.54 in failed patients and was 26.42 in successful mapping patients ( p = 0.042). Surgeons should be aware that node counts will decrease with increasing age and that increased age and BMI are potential risk factors for SLN mapping failure. However, increased age and/or BMI alone do not appear to be contraindications for SLN biopsy in older or overweight patients.