Patricia Grainger was strangled, stabbed and sexually assaulted near her home in Parson Cross, Sheffield, on 10 August 1997.
Her body was dumped in a brook in nearby woodland and her killer has never been found.
Her son, Daniel, 23, said South Yorkshire Police is not doing enough to find his mother's attacker.
Mr Grainger told BBC Radio Sheffield: "I want the police to do more. It's not an active investigation.
"I don't want 100 officers looking at it, I just want someone to say, 'We're going to try to find this person'.
"My mother was murdered, she was a victim and the person [that did that] is still out there.
"I want that justice served."
Mr Grainger set up a website, whokilledmymum.com, when he was 16 years old appealing for information.
The force said the investigation is "subject to ongoing review" but it no longer has some of the evidence related to the case.
Ch Supt Rachel Barber said a complaint from Mr Grainger is being addressed and new lines of inquiry would be followed up.
"The items of clothing Mrs Grainger was wearing at the time of her death were submitted to the Forensic Science Service and were subject to a thorough examination. As a result, South Yorkshire Police is no longer in possession of these items," she added.
The BBC asked South Yorkshire Police if it had followed correct procedure in sending evidence to the FSS, which closed in March 2012, but the force did not provide a comment.
The son of a murder victim has criticised police on the 18th anniversary of his mother's death.