Recently, a relationship was demonstrated between the thickness of the cementum layer in rat molars and the activity of alkaline phosphatase (ALP) in the adjoining periodontal ligament (PDL). It was the aim of the present study to investigate whether such a relationship also exists in the periodontium of man. Healthy deciduous and permanent teeth free from periodontitis were obtained from 74 patients, varying in age from 3 to 78 yr, and their PDL dissected from the middle one-third of the roots. ALP activity was measured in PDL extracts and expressed per hydroxyproline content. It was shown that ALP activity was relatively high in children. After puberty its concentration decreased to level off at about half the concentration found in the younger age groups. The activity of the enzyme in the PDL correlated positively with the yearly cementum thickness increment as calculated from data published previously.