A woman was found with a serious head injury in Lancing on Tuesday night.
Police are trying to find out what happened to the 48-year-old, who was found collapsed in the Westway area at about 9pm by members of the public.
An ambulance was called and she was admitted to Worthing Hospital, where she is still undergoing treatment.
Police would like to hear from anyone who saw what happened to the woman as her recall is currently unclear.
Anyone who has information is asked to phone Sussex Police on 101 or email firstname.lastname@example.org, quoting serial 235 of 09/7.