Background Research has demonstrated a relationship between the experience of life events and psychopathology in people with intellectual disabilities (ID), however few studies have established causal links and to date no prospective studies have utilised a measure of trauma that has been developed specifically for this population group. Method This 6-month prospective study examined longitudinal relationships between adverse life events and trauma in 99 adults with mild to moderate ID. Results Life events during the previous 6 months were significantly predictive of levels of trauma as measured by the self-report Lancaster and Northgate trauma scales (LANTS), and the informant LANTS behavioural changes, frequency and severity sub-scales. This prospective causal relationship was demonstrated while controlling for any prior life events or pre-existing trauma, though the relationship was not moderated by social support. Conclusions Evidence of a causal relationship between adverse life events and trauma symptoms is important for treatment planning and funding allocation.