Aims To verify the applicability, reproducibility and predictive value of a proposed unified classification (amended Ljubljana classification) for laryngeal squamous intraepithelial lesions (SILs). Methods and results Six internationally recognized experts and three pathologists from Ljubljana contributed to this study by evaluating a set of laryngeal SILs using the new system: low‐grade SIL, high‐grade SIL, and carcinoma in situ (CIS). The overall agreement among reviewers was good. Overall unweighted and weighted κ‐values and 95% confidence intervals were 0.75 (0.65–0.84) and 0.80 (0.71–0.87), respectively. The results were stratified between the international reviewers and the Ljubljana pathologists. The former had good overall agreement, and the latter had very good agreement. Kaplan–Meier survival curves showed a significant difference (P < 0.0001) between patients with low‐grade and high‐grade SILs; 19 of 1204 patients with low‐grade SILs and 30 of 240 patients with high‐grade SILs progressed to malignancy in 2–15 years and in 2–26 years, respectively. Conclusions The proposed modification to the Ljubljana classification provides clear morphological criteria for defining the prognostic groups. The criteria facilitate better interobserver agreement than previous systems, and the retrospective follow‐up study demonstrates a highly significant difference in the risk of malignant progression between low‐grade and high‐grade SILs.