Assessment of child development is mainly determined by characteristics of its spontaneous behaviour. Therefore, during the assessment of child development, it is important to notice, observe and evaluate which caracteristics indicate on typical or atypical development. Considering the fact that the assessment of spontaneous behaviour is one of the crucial factors in diagnostics of autism spectrum disorder, it is important to compare and try to discriminate typical and atypical behaviours in early age. The aim of this research was to examine differences in people’s subjective assessment of spontaneous behaviour of threeyear-olds and determine possible factors that could contribute to those differences. The specific aims included assembling a questionnaire and comparing subjective assessments of participants considering their age, profession and information whether they are parents or not. The questionnaire contains 25 closed-ended questions (claims) on which participants on Likert scale mark to what extent they find that some behaviour is (a)typical. Considered behaviours were: stereotyped use of motor movements, objects or speech, inflexible adherence to routines and insistence on sameness, highly restricted and fixated interests, hyper- or hyporeactivity to sensory input, characteristics of social communication and social interaction. The questionnaire in online form was filled by 205 participants. The results indicated statistically significant differences between different groups of participants. Younger and older age group differ in assessments in the way that younger group consider certain behaviour more atypical than older group. Differences considering profession indicate that experts working with children with disabilities estimate many behaviours as atypical in comparison to experts working in education system or all other participants. Likewise, participants with children estimate certain behaviour more atypical than participants with no children. Despite the existence of methodological limitations which prevent the generalization of results and determination of all factors that could contribute to the differences in participants’ assessments, collected data could contribute to the distinction beetwen typical and atypical behaviour of three-year-olds. Considering that conducted research is one of the few that have been examined subjective assessment of children behaviour, gained knowledge will enrich and instigate future research of this domain.