A random sample of 35-year-old subjects from Oslo took part in a dental survey in 1973 and were re-examined in 1988. Eighty-one subjects (85%) attended the final examination. The need for periodontal treatment was assessed by the Periodontal Treatment Need System (PTNS), and the oral hygiene by the Simplified Oral Hygiene Index (OHI-S). The participants attended a structured interview and answered a questionnaire about general and dental health habits as well as psycho-social factors. Only small changes in the distribution of subjects in the different PTNS categories were found to have taken place during the 15 years. In 1973, 56.8% were in need of scaling (Class B) and 32.1% had one or more deep inflamed pockets (Class C), and in 1988 the scores were 54.3% and 30.1% respectively. A logistic regression model was used to study the associations between risk factors and increased treatment need, as expressed by increase in the number of C-quadrants. Increased number of C-quadrants was positively associated both with short duration of education and with no interdental cleaning. Using a socio-ecological model for periodontal diseases, variables describing the items "behaviour" and "environment" were found to be most closely associated with increased need for periodontal treatment.