Arrested fly ash samples from most currently operating municipal solid waste (MSW) incinerators in the UK have been analysed for a range of elements. Some of the more important heavy metals ranged in concentration as follows: Cd, 21–4646 (median = 271) mg kg−1; Cu, 296–1307 (642) mg kg−1; Cr, 44–1328 (574) mg kg−1; Ni, 45–2204 (74) mg kg−1; Pb, 447–9704 (4337) mg kg−1; and Zn, 2285-13 500 (9232) mg kg−1. These concentrations represent considerable enrichments relative to median UK soil concentrations. Enrichment ratios (defined as median fly ash: median UK soil) were as follows: Mn 1·6; Co 2·6; Ni 3·3; Ba 11; Sr 11; Cr 15; Cu 35; Pb 108; Zn 113; Cd 387. It is estimated that MSW incinerator ash contributes c. 15 t Cd and 241 t Pb to UK landfill sites per annum. These figures compare with previous studies by Hutton & Symon (Hutton, M. & Symon, C. (1986). The quantities of cadmium, lead, mercury and arsenic entering the UK environment from human activities. Sci. Total Environ., 57, 129-50.) which estimated that annual inputs to UK landfills from coal fly ash are c. 60 t Cd and 1270 t Pb. However, it is argued that metals associated with MSW ashes are potentially of greater environmental significance than in coal ashes, because they are much more available and present at much higher concentrations.