A study was undertaken to estimate the radial diffusion timescale, τ _LL , for relativistic electrons (2–6 MeV) to diffuse into the slot region due to drift-resonance with Pc5 ULF waves (2–10 mHz) on 29 October 2003. Large amplitude ULF waves were observed by ground-based magnetometer arrays to penetrate deep into the slot region (L 2–3) starting at 0600 UT and maximising (∼200 nT p-p) between 0930–1630 UT. Around the same time, the SAMPEX PET instrument measured an over two orders of magnitude increase in relativistic (2–6 MeV) electron flux levels in ∼24 hours within the slot region. The ground-based D-component magnetic power spectral densities (PSD_δB ) for 29 October were estimated for six latitudinally spaced ground stations covering L ∼ 2.3–4.3 for an observed ULF wave with central frequency ∼4 mHz. The PSD_δB values were used to calculate the in situ equatorial poloidal wave electric field power spectral densities (PSD_δEm ) using a standing Alfvén wave model. The radial diffusion coefficients, D _LL , were estimated using the PSD_δEm values. The fastest τ _LL were 3–5 hours at L > 4, while τ _LL initially increased with decreasing L-value below L 4; peaking at L 3 with τ _LL ∼ 12–24 hours with PSD_δEm estimated using a wave frequency bandwidth between Δf = 1 mHz and Δf = 2.5 mHz. The τ _LL over the L-range L ∼ 2.3–3.3 were consistent with the timescales observed by SAMPEX for the increase in relativistic fluxes in the slot region on 29 October. The authors believe that this is the first example of the ULF wave drift-resonance with relativistic electrons explaining a radiation belt slot region filling event.