The analyses presented in the article aim to investigate the specific nature of visual problems in a young child whose visual disability results from a brain tumor. The article presents a case study on difficulties in using vision and visual perception development in an almost 4-year-old boy with a brain tumor. It refers to knowledge concerning visual problems in children with brain tumors that is available in source materials and presents a detailed description of difficulties in visual reception and perception in a boy whose visual problems result from cancer. This description was made based on the results of a functional vision assessment. Decreased visual acuity, reduced visual fields, abnormalities in the development of oculomotor functions, lack of spatial vision, and difficulties in visual perception were found in the boy. Both the analysis of medical literature presented in the paper and the results of the boy's functional vision assessment suggest possible development of visual functioning disorders secondary to a brain tumor, such as decreased visual acuity and reduced visual fields, which determine visual abilities and have an adverse impact on the development of visual perception in early life. Based on the analysis of an individual situation, the description of difficulties in visual functioning suggests there is a risk of similar impairments in children with brain tumors.