A regional epidemiologic investigation comprising one-tenth of the Danish population showed that from 1963 to 1968 the annual incidence of new duodenal ulcer cases averaged 1.4 per 1000 inhabitants aged 15 years and over. The corresponding incidence of new cases in previous "non-ulcer" patients was 1.3 per 1000 inhabitants. Similar incidence rates have been registered in England and Norway, while rates from Scotland are much higher. The total frequency and age fractions were constant during the period. The male:female ratio was 2.2 to 1, but varied with the morphologic base of the diagnosis. Thus, the sex ratio for pyloric ulcers was 1:1. The sex ratio generally was not influenced by the patient's age. The age-specific incidence rates increased in an almost linear manner in men and in women from the age of 15 to the maximum age of 75-79 years, where it reached 3 per 1000. The clinical debut of duodenal ulcer in males showed conversely an equal incidence in all age groups over 20 years. In females the corresponding probability rates increased until 40 years of age, after which they remained constant.