There has been a long-term decline, causing frequent local extinctions, of cyclic grey-sided voles (Clethrionomys rufocanus) in northern Fennoscandia since 1971. Previous studies supported the hypothesis that altered landscape structure, especially in terms of forest patch area and fragmentation of oldgrowth forest, has contributed to the decline. Since those studies were based on cumulated vole time series data and static landscape structure, we now tested whether the long-term decline was related to a gradual change. We digitized landcover types (>0.25 ha) from aerial photographs within 6.25 km2 squares centred on each of the 27 sampling sites with 5 year intervals, starting in 1970. Because of clear-cutting, mean area of the patches of >35 year old forest that intersected the sampling sites decreased from 126 ha in 1970 to 44 ha in 2004. The main decrease in focal forest patch area occurred in 198085, coinciding with the major drop in vole numbers. Our results strongly suggest that long-term habitat fragmentation is involved in the current decline of grey-sided voles. However, climate change leading to warmer winters with a less stable snow cover is also thought to be of major importance, as indicated by a decrease in vole wintering success.