The aim of the study was to find out if any of the following factors, alone or in combination, could be used for prediction of caries activity: salivary S. mutans, salivary lactobacilli, buffer capacity, saliva secretion rate, salivary pH, the amount of plaque and the caries frequency. The patient material consisted of 300 children between 9 and 12 years old. The increment of caries after 1 and 2 years was compared with all the above factors registered at baseline. Only the factors caries frequency and S. mutans showed a significant correlation to caries activity. In one of the tests the lactobacillus count approached significance. The combination of high numbers of incipient smooth surface lesions, S. mutans and lactobacilli gave a mean value of caries activity almost three times that of the whole group (8.4 versus 3.1).