The dental decay activity of a patient can be evaluated with the help of certain indirect signs that reveal the dental decay risk elements. The quantity, buffer capacity, inorganic components of the saliva are important in the pathogenesis of dental decay. The micro-crystallization saliva test (IMK) may represent an important criterion on whose basis one obtains an evaluation of the re-mineralization capacity of saliva. The purpose of this study consisted in the calculation of the IMK index on a lot of patients and its correlation to the type of dental decay activity. The study was conducted on a lot of 30 patients with ages between 18 and 45, nursed and examined every 6 months during 18 months. The attention was focused on the following parameters: the index of micro-crystallization (IMK) and the dental decay activity, calculated as rate of appearance of new dental lesions within 18 months. As a result of investigations, the authors observed the following correlations to dental decay activity: the patients with IMK = 1/0.6 did not have any new dental lesion, in the patients with IMK = 0.6/0.4 new dental lesions appeared within 12-18 months and in the patients with IMK = 0.4/0 new dental lesions appeared in less than 6 months after examination. The IMK index--indicator of the re-mineralization capacity of saliva, may be evaluated using a relatively simple methodology, accessible and at a moderate price. Research pointed out that this index may be an useful instrument when evaluating the dental decay susceptibility of patients, contributing to the identification of the dental decay risk and consecutively to the choice of individualized prophylactic and therapeutic procedures.