Abstract The effects of neutron irradiation on beryllium in terms of helium generation and induced swelling have been studied. Beryllium samples were irradiated in the Belgian materials testing reactor BR2 at low temperature (40–50 °C) up to 20 000 appm He (for a fission neutron fluence of 5.3 × 10 22 n cm −2). Post-irradiation annealing was performed to study the effect of temperature on the helium behaviour and the induced swelling. Macroscopic examinations of the gas content revealed a linear correlation of 4He content with fast neutron fluence. 4He contents were found to be about 10% higher than their expected formation as calculated from the various formation schemes. The 3He formation was also measured. The 3He calculations have to take into account the continuous transition between 3He and 3H according to the scheme the latter reaction occurring only when the reactor is in operation. Macroscopic examinations of the swelling by density measurements revealed a linear correlation of low temperature irradiation-induced swelling with He content. This swelling was found to be strongly enhanced after post-irradiation annealing at temperatures above 250 °C. The enhancement of the swelling could be correlated with (He content) and with (annealing temperature). Microscopic examinations on the samples have been started to elucidate the behaviour of He in beryllium. Preliminary results are given.