Coldharbour hospital in Sherborne was built during the Second World War as an orthopaedic hospital for injured Navy personnel. It came under the NHS after it was founded in 1948.
A fire broke out at 2.30am on 5 July 1972, killing 30 patients, mostly by smoke inhalation, as they slept.
At the time of the tragedy the hospital housed around 350 patients with learning disabilities.
The memorial services being held at the cemetery and Sherborne Abbey are to commemorate and remember those that sadly died.