The aims of this study were (i) to determine the reasons for extraction of permanent teeth in Norway in 1998 and (ii) to describe changes in reasons for extraction in time-trend and quasi-longitudinal perspectives over 30 years based on three cross-sectional studies. The 1998 data were obtained from questionnaires distributed to a national random sample of 1,500 dentists, of whom 1,008 responded (67%). A total of 1,495 teeth were extracted for 1,164 patients (age 10-92 years) during a period of 2 weeks. Caries accounted for 40% of the extractions and periodontal diseases for 24%. Caries was the primary reason in patients 10-45 years old, while periodontal diseases dominated among adults > 45 years old. In a time-trend perspective, among patients > or = 21 years, a significant decrease in the proportion of extractions because of caries was observed between 1968 and 1988, while non-significant changes were found between 1988 and 1998. Extractions because of periodontal disease accounted for about 26% in all surveys. Followed longitudinally, the three cohorts born 1923-32, 1933-42, 1943-52 showed a decrease in the proportion of extractions due to caries, and an increase in the proportion of periodontal extractions during the period, more so between 1968 and 1988 than between 1988 and 1998. The three cohorts showed a variable trend of change, which may indicate that period and age effects do not necessarily operate in the same direction.