The purpose of the present investigation was to study the distribution of interproximal periodontal intrabony defects as related to age, sex and localization, and to examine the relationship between some possible etiological factors and the occurrence of intrabony defects. A total of 209 adult patients presenting for dental treatment at the Royal Dental College in Arhus participated in this study. The patients were subjected to a questionnaire and were examined clinically and radiographically. All osseous defects in the radiographs were recorded but only those with a depth and width of a least 2 mm were considered periodontal intrabony defects. Only 18% of the participants had one or more periodontal intrabony defects but the prevalence was higher in older than in younger age groups. While periodontal intrabony defects occurred with the same frequency on the various tooth types, more defects were found on distal than on mesial surfaces. The presence of periodontal intrabony defects correlated with loss of attachment, increased tooth mobility, a wide interproximal space and open mesio-distal contact relationships between the teeth.