Single-walled carbon nanotubes (SWCNTs) have recently attracted great attention because of their fibrous structure and high aspect ratio. Here the genotoxic potential of 400-800 nm, 1-3 μm and 5-30 μm SWCNT with respect to their geometry and surface characteristics was studied. Following thorough physico-chemical characterisation, human bronchial epithelial (BEAS-2B) and lymphoblastoid (MCL-5) cells were treated with SWCNT for 24 or 48 h. This showed significant increases in micronucleus frequency in a time- and dose-dependent manner in both cell types in the absence of cytotoxicity. Over the same dose range, only 1-3 μm SWCNT gave rise to significant increases in hprt point mutations at doses ≥25 μg/ml. Cellular 2,7-dichlorodihydrofluoresceindiacetate (DCFH-DA) fluorescence assay and RT-PCR for oxidative pathway gene profiling revealed a possible oxidative mechanism for the genotoxicity observed in the 1-3 μm SWCNT. Consequently, this study has demonstrated that SWCNT genotoxicity is dependent on its secondary structure under experimental conditions and oxidative stress alone cannot account for the observed damage.