Police and State Emergency Services are searching for a 46-year-old man missing in dense bushland 70km north-east of Menzies.
Michael Keith Graham was last seen at 10pm on Sunday while camping with his friend Peter Hollaway. When Mr Hollaway woke the next morning, Mr Graham had vanished.
It is believed he wandered away during the night. He was last seen wearing a grey jumper and shorts. Incident controller Sergeant Mark Mariott said Mr Graham had left no indication of his direction of travel and had taken no water or navigation equipment.
He has had both knees replaced, a kidney removed and has been on painkillers.
He said search crews had found tissues in trees potentially indicating a trail laid by Mr Graham. Police are searching a 100sqkm region of undulating hills crossed with dry creek beds. About 20 SES volunteers have been mobilised from Kalgoorlie-Boulder, Norseman, Leonora, Laverton, Menzies and Perth.
Department of Fire and Emergency Services liaison officer Colin Brown said the operation would escalate through to the weekend and may involve up to 50 SES volunteers.