Heavy metals in municipal solid waste incineration bottom ash (MSWIBA) may leach into soil and groundwater and pose long-term risks to the environment. In this study, toxicity characteristic leaching procedure (TCLP) was carried out on the MSWIBA from Macao. Heavy metals in leachates were determined by inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma atomic emission spectrometry (ICP-AES), and genotoxicity of leachates was also evaluated by micronucleus (MN) assay with Vicia faba root tip cells. The results showed that the concentrations of aluminium (Al), manganese (Mn), cobalt (Co), cadmium (Cd) and mercury (Hg) in the leachates were less than 0.01 mg l<sup>-1</sup>, and those of iron (Fe), copper (Cu) and molybdenum (Mo) were less than 0.1 mg l<sup>-1</sup>. The concentrations of chromium (Cr), zinc (Zn), selemium (Se), strontium (Sr), barium (Ba) and caesium (Cs) were between 0.11 mg l<sup>-1</sup> and 2.19 mg l<sup>-1</sup>. Lead (Pb) concentrations, in particular, reached as high as 19.6 mg l<sup>-1</sup>, significantly exceeding the maximum concentration limit (5 mg l<sup>-1</sup> for lead by TCLP). Compared with the negative group, a significant increase of MN frequencies was observed in the leachate-exposed groups (P <0.05). With the increase of heavy metals in the leachates, the toxic effects on the Vicia faba root tip cells increased, implying that heavy metals were the main factors causing the genotoxic effects. Our results suggested that apart from chemical analysis, bioassays like the MN assay of Vicia faba root tip cells should also be included in a battery of tests to assess the eco-environmental risks of bottom ashes before decisions can be made on the utilization, treatment or disposal. ©2006 Elsevier Ltd. All rights reserved.