Mean sulphur dioxide levels have fallen markedly in London, UK during the past 15 yr. Although lichens growing on trees are particularly sensitive to this air pollutant, there are few studies of lichen recolonization following pollution episodes. A survey of 29 sites in the north and west of Greater London, reported here, demonstrates that several species extinct or very rare in the area in 1970 have extended their ranges considerably. Studies on growth rates of the species concerned suggest that many of the sites discovered have been recolonized within the past 3-7 yr. If current trends continue, further improvements in the lichen flora can be expected in the next few years. However, it is unlikely that London will regain in the forseeable future many of the species lost during the past two centuries.