Solid state nuclear track detectors were used in a regional survey of radon in indoor air. The study area comprises seven rural municipalities and two towns in an area of 80 X 50 km2 with a population of about 65,000. Measurements were made in 754 houses in 31 subareas. The highest and lowest subarea means were 1,200 Bq/m3 and 95 Bq/m3, respectively. The estimated mean for the whole area was 370 Bq/m3. The concentrations 2,000 Bq/m3 and 800 Bq/m3 were exceeded in 32 and 90 houses, respectively. The present lung cancer incidence does not differ significantly from the national mean.