The average indoor radon concentration in Finnish flats is 80 Bq/m3. Typically walls have been madeusing concrete elements. Building materials are the dominant source of indoor radon. However, in theflats of the lowest floor, with a floor slab in direct ground contact, the main source of radon is oftenthe inflow of radon bearing soil air. The number of these ground contact flats is less than 10 % of thetotal number of flats. The average indoor radon concentration of these ground contact flats is 150Bq/m3. Consequently in more than 10 % of these dwellings the concentration limit of 400 Bq/m3 isexceeded. The data presented is based on a nation-wide radon survey in 900 flats. Highestconcentrations observed in flats have exceeded 10.000 Bq/m3.Best mitigation results were achieved using radon wells and sub-slab-depressurisation, the typicalradon concentration reductions being 50-80 %. High depressures in flats decrease the efficiency ofboth radon wells and sub-slab-suction installations. In comparison, the radon concentration reductionsfor radon wells, obtained in low rise residential houses where depressures are lower, were typicallyremarkably higher, 80-95 %.The exhaust ventilation, in combination with the air tightness of the concrete elements and windows,causes remarkable underpressures over the floor slab, increasing the inflow of radon-bearing air fromsoil. In older flats no fresh air vents have been required. Installation of fresh air vents has been anefficient mitigation method in flats. In the cases studied radon reduction rates exceeding 50 % havebeen achieved.Sealing entry routes has typically resulted in varying reduction rates of 30-60 %. In order to achievegood results a rather complete sealing of the gap between the wall and floor slab should be performed.However, in many cases a complete sealing work is tedious and expensive.On the basis of the results achieved, it is recommended that a case-specific analysis of the installationof fresh-air vents and entry route sealing should be first considered for radon remedies of flats. If thesemethods are not adequate, sub-slab-suction and radon well should be considered.