Regressive autism occurs when a child appears to develop typically but then starts to lose speech and social skills at about the age of 18 months and is later diagnosed with autism. Other terms used to describe regression in children with autism are autism with regression, autistic regression, setback-type autism, and acquired autistic syndrome. There is no standard definition for regression, and the prevalence of regression varies depending on the definition used.

Approximately 25–30% of children with autism spectrum disorders stop speaking after beginning to say words, often before the age of two. Some children lose social development instead of language; some lose both. After the regression, the child follows the standard pattern of autistic neurological development. The term refers to the appearance that neurological development has reversed; it is actually only the affected developmental skills, rather than the neurology as a whole, that regresses. It is more usual for autistic neurological development to not include such aberrations, with age-appropriate autistic symptoms being clear from birth.

Skill loss may be quite rapid, or may be slow and preceded by a lengthy period of no skill progression; the loss may be accompanied by reduced social play or increased irritability. The temporarily acquired skills typically amount to a few words of spoken language, and may include some rudimentary social perception. In some cases a child will go through a period of learning new words at the cost of losing previously acquired words.

Regression in autism spectrum disorders is well documented; attributing regression to environmental stressors may result in a delay in diagnosis. The apparent onset of regressive autism is surprising and distressing to parents, who often initially suspect severe hearing loss. The pattern of symptoms naturally leads many to suspect that postnatal environmental factors trigger autism. In particular, because obvious symptoms start just after children receive multiple vaccinations, such as MMR plus varicella (chickenpox), some people perceive a causal link between vaccination and autism, especially if accompanied by a fever, rash, and rapid skill loss. Although some controversy remains no link has been found with vaccines.

Other disorders that involve regression are total blindness from birth, childhood disintegrative disorder, Rett syndrome and Landau-Kleffner syndrome.