The aim of this work was to identify the choroideremia families in northern Finland, form an impression of the incidence of the disease in Finland, construct a picture of its clinical progression and gather new information on relevant genetic questions. A total of 111 choroideremia patients and 188 carriers were traced, members of four families from northern Finland and one from the Savo district. Ophthalmological confirmation was obtained for 84 choroideremia cases and 126 carriers. The largest of the families from northern Finland contained 80 cases of the disease and 146 carriers in eight generations among a total of more than 3000 descendants from one ancestral mother. The clinical picture for choroideremia proved to be more variable than could have been supposed from the literature, including cases of patients under 30 years of age who were already virtually blind and of patients of over 50 who were subjectively symptom-free. Only 7 out of 105 carriers could be shown anamnestically to have had subjective symptoms, but surprisingly, as many as 21 out of 52 carriers examined had changes in the visual field and 13 out of 40 examined showed deterioration in dark adaptation. One carrier was seen to undergo an obvious decline in dark adaptation during a three-year observation period. One indirect indication of the progression of fundus changes in choroideremia carriers was obtained from the fact that these changes, and also alterations in visual field and dark adaptation, were greater in the older carriers. A progression could also be detected by fundus photography in six instances, although the changes involved were fairly mild ones. Considerable variety was noted in the fundus findings for the choroideremia carriers, there being some 80-year-old subjects with quite minor changes and some 20-year-olds with obvious, extensive changes. Practical visual acuity remained normal throughout life in the majority of the carriers, however. Diagnosis within the known choroideremia families was fairly difficult, especially at the early stages in the survey, and even later on a few cases aged up to ten years produced diagnostic problems. Quite often diagnosis was easy, however, and choroideremic fundus changes were even identified in two boys aged 3 and 8 months. No other diseases could be shown to be associated with choroideremia, and the occurrence of dominantly inherited olivopontocerebellar atrophy alongside choroideremia in one branch of a family may be regarded as a coincidence.(ABSTRACT TRUNCATED AT 400 WORDS)