Autism spectrum disorder (ASD) is classified as a neurodevelopmental disorder with an increasingly high incidence rate. Increasing awareness, changing diagnostic criteria and social attitudes, as well as financial considerations, will affect the prevalence of diagnosis. The symptoms of ASD vary widely, making it difficult to detect. It represents a spectrum of alterations ranging from mild indications to severe impairments and diagnosis is based on a very rigorous behavioral assessment. Nevertheless, certain neuropathological changes are common, although the background of this disorder remains still unknown. Therefore, some research aimed at better understanding the pathology of the neurological alterations in ASD, as well as the possibilities for early diagnosis and treatment of this disorder, is urgently needed. This review summarizes the current evidence on some selected proteins such as tau protein, NFL, and BDNF as well as IGF-1 that appear to be the best protein candidates for better understanding the causes of autism, as well as for use as fluid biomarkers in the diagnosis and monitoring of ASD.