[en] In JT-60U, extensive plasma-neutral interaction during type I edge localized modes (ELMs) leads to a transient increase in the edge plasma density, seen as spikes of a few ms duration on the inner vertical interferometer channel (FIR1). The spikes can reach up to ∼40% of the pre-ELM level of the FIR1 signal, in the case of giant ELMs. Fast edge measurements revealed that these density spikes are caused by ionisation of neutrals circulating between the plasma and the wall, as a result of the ELM particle and heat load onto the target. The increase in the edge density is more or less equally divided between the scrape-off layer (SOL) and outer core region, but is outside of the top of the H-mode pedestal. Prompt ionisation of neutrals and the increase in the plasma density around the separatrix position may affect the edge MHD stability, as often manifested by the occurrence of a second, satellite ELM triggered at lower pedestal temperature and pressure, clusters of type III ELMs or periods of L-mode. Both the magnitude of the density spikes and the observed changes in the ELM behaviour were found to depend on the wall conditions