This study assesses the incidence and prevalence of cryptogenic fibrosing alveolitis (CFA) in a well-defined and stable Norwegian population of 250,000 inhabitants during a period of 15 years. We conducted a file survey of all patients (n = 376) aged 16 years or older with a clinician's diagnosis of pulmonary fibrosis (ICD 8: 517 and ICD 9: 515 and 516). Cases with a history of exposure to fibrogenic agents or with collagen vascular disease were excluded and the remaining 158 cases were defined as CFA. The average annual incidence of hospitalised CFA was 4.3 per 100,000. No change was observed in the annual incidence during the 15-year study period. The prevalence was 19.7 and 23.9 per 100,000 by 31.12. 1991 and by 31.12. 1998, respectively. The incidence increased considerably and significantly with age. No significant gender differences were observed. The lack of gender differences and the increase with age were also found when the diagnosis of CFA was exclusively based on cases with hospital file records of breathlessness, bilateral crackles and bilateral shadows on chest radiograph.