Context.— Allograft liver biopsy is the gold standard in assessing transplant recipients for graft dysfunction. The impact of biopsy sample size on the diagnosis of acute cellular rejection (ACR) has not been studied. Objective.— To assess the relationship of biopsy sample length with the diagnosis and determine optimal biopsy sample size in the transplant setting. Design.— We retrospectively reviewed 68 core biopsies from patients with history of liver transplant. Each biopsy sample was read, on 5 different occasions with differing lengths, to assess for ACR per Banff criteria. Categorical agreement was calculated from rejection severity. Results.— The length of biopsy sample strongly correlated with the number of portal tracts. ACR rates increased from 73.5% to 79.4% with increase in length from 1 cm to 2 cm and moderate rejection increased from 27.9% to 33.82%. At 1.0 and 1.5 cm, no cases of severe rejection were detected; at 2.0 cm, 1 case; and at 3.0 cm, 2 cases were detected. The major error rate was reduced to less than 10% with a length of 2.0 cm, at which length the average number of complete and partial portal triads was 10 and 13, respectively. Conclusions.— The likelihood of diagnosing ACR and rejection grade increased substantially with increase in biopsy sample length. This study suggests that a minimum length of 2 cm, 10 complete portal triads, or 13 partial/complete portal triads should be obtained for confident exclusion and grading of ACR.