AbstractIn the 2021 WHO thoracic tumors, gradation of lung carcinoids in biopsies is discouraged. We hypothesized that atypical carcinoid (AC) could be reliably diagnosed in larger preoperative biopsies. Biopsy-resection paired specimens of carcinoid patients were included, and definitive diagnosis was based on the resection specimen according to the WHO 2021 classification. A total of 64 biopsy-resection pairs (26 typical carcinoid (TC) (41%) and 38 AC (59%)) were analyzed. In 35 patients (55%), tumor classification between the biopsy and resection specimen was concordant (26 TC, 9 AC). The discordance in the remaining 29 biopsies (45%, 29 TC, 0 AC) was caused by misclassification of AC as TC. In biopsies measuring < 4 mm², 15/15 AC (100%) were misclassified compared to 14/23 AC (61%) of biopsies ≥ 4 mm². Categorical concordance of Ki-67 in biopsy-resection pairs at threshold of 5% was 68%. Ki-67 in the biopsy was not of additional value to discriminate between TC and AC, irrespective of the biopsy size. Atypical carcinoid is frequently missed in small bronchial biopsies (< 4 mm²). If the carcinoid classification is clinically relevant, a cumulative biopsy size of at least 4 mm² should be considered. Our study provides strong arguments to make the diagnosis of AC in case of sufficient mitosis for AC on a biopsy and keep the diagnosis “carcinoid NOS” for carcinoids with ≤ 1 mitosis per 2 mm². Ki-67 has a good concordance but was not discriminative for definitive diagnosis.