The effect of internal hydrogen on the formation of vacancy dislocation loops from heavy-ion generated displacement cascades in aluminum has been investigated by using transmission electron microscopy. Samples of high-punty aluminum and aluminum containing 900 and 1300 appm (atomic parts per million) of hydrogen were irradiated at room temperature with 50-keV Kr⁺ ions. Under these irradiation conditions, dislocation loops were observed in all compositions, although the formation probability was relatively low (less than 10% of the displacement cascades created a vacancy loop). The loop formation probability was further reduced by the presence of hydrogen. No difference in the geometry or the size of the loops created in the hydrogen-free and hydrogen-charged samples was found.