An official at the renowned brain bank in the McLean Hospital, Massachusetts discovered that the freezer had shut down in late May, but did not trigger warning alarms.
The hospital authorities later found 150 thawed brains that had turned dark from decay; about a third of them were part of a collection of autism brains, reported the Boston Globe newspaper.
Autism is a developmental disorder that appears in the first 3 years of life, and affects the brain's normal development of social and communication skills, according to US National Library of Medicine.
"This was a priceless collection. You can't express its value in dollar amounts," said Dr Francine Benes, director of the Harvard Brain Tissue Resource Center.
The loss of the brain tissue could set back autism research by a decade, according to Carlos Pardo, associate professor of neurology at Johns Hopkins University.
He said the brains, "yield very, very important information that allows us to have a better understanding of what autism is, as well as the contribution of environmental and immune factors."
The freezer was thought to have failed three days before it was discovered, but alarms failed to trigger and an external thermostat gave the incorrect temperature, the report said.
It contained 150 brain samples from people who had neurological conditions including autism, Parkinson's disease and schizophrenia. PTI