AIMS: Although high-grade dysplasia (HGD) is a risk factor for malignant transformation and future development of adenomas/carcinomas, grade is not incorporated in the Dutch guidelines for colonoscopy surveillance, partly due to presumed interobserver variability. In a nationwide cohort of colorectal adenomas, we analysed the interlaboratory variability in the grading of dysplasia in daily practice. METHODS AND RESULTS: Using the Dutch Pathology Registry (PALGA), all synoptically reported classic adenomas in The Netherlands in 2013 were identified. Proportion of adenomas with HGD was determined for biopsies and polypectomies and compared between 37 laboratories by multivariable logistic regression analyses. In total, 21,030 colonoscopies of 20,270 patients were included. HGD was reported in 530 (3.6%) out of 14,866 adenomas diagnosed on biopsies (range between laboratories: 0%-13.6%) and in 983 (11.8%) out of 8,346 adenomas diagnosed on polypectomies (range 3.1%-42.9%). After adjustment for case mix, thirteen (35%) laboratories reported a significantly lower or higher frequency of HGD than average. CONCLUSIONS: We observed considerable interlaboratory variation in the grading of dysplasia in colorectal adenomas, which could only be partly explained by differences in case mix. Therefore, better standardization of grading criteria is needed before grade of dysplasia can usefully be incorporated in colonoscopy surveillance guidelines.