Purpose: The aim of this study was to propose a new clincial diagnostic rating index for secondary caries lesions at crown margins and to correlate this index with histologic evaluations. Materials and Methods: Based on criteria for the evaluation of root caries, a modification for secondary caries lesions at the crown margin (SC index) was applied; the lesion characteristics were described as SCO to SC4. A total of 16 crowned teeth exhibiting secondary caries were randomly selected. The teeth were rated and grouped according to their clinical SC grades (SC1 to SC4). After embedding in Technovit, the teeth were serially sectioned and histologic caries scores were given for each section. Results: Fifty-two percent of the prepared margins were in cementum/dentin. Crown margins with no caries lesions were mainly found in teeth clinically rated as SC1 and exhibiting localized discolorations, whereas deep lesions were found histologically at teeth rated SC4. The nonparametric test according to Spearman's rank correlation coefficient (rho) confirmed a highly significant correlation between the clinical SC index and the histologic evaluation (rho = 0.87, 95% confidence interval = 0.67 to 0.96, P 0.01). SC1 discolorations overestimate caries lesions, whereas SC2 and SC4 lesions are well correlated to the percentage of histologically evaluated caries extension into all sections of a tooth. Lesions rated SC3 demonstrated a high variability of caries penetration into the dentin. Conclusion: The depth of discolorations and circular lesions is clinically not well predictable. Clinical diagnosis for localized superficial caries and deep caries lesions at the crown margin correlated well with the histologic caries rating.