The article presents the results of experimental studies of the psycho-diagnostic parent-child relationship in 25 families with preschool children (5-6 years), who are patients of acute lymphoblastic leukaemia. The study was conducted using the following methods: PARI, (Schaefer and Bell); "The analysis of family relationships," (Eidemiller and Yustitskis); Rene Gilles, aimed to study the sphere of interpersonal relations of the child and his perception of family relations -"Kinetic family picture," as modified by Burns and Kaufman. The results of our study have shown that 'psychologically affluent' families were ones where affected children (boys) were in a relationship with their families and siblings.The families of the girls indicated "phobia of losing a child" and "conflict between the parents." In families with only one child predominantly, there were "conflict between parents", "emergency intervention in the child's life," and "projection on the child's own undesirable qualities." The threat of the loss of an only child, of course, is a very painful experience for parents, causing a lot of negative emotions and conditions. Analysis of the relationship between the studied parameters in single-parent families has shown that the more a mother takes care of her seriously ill child, the less the child needs to communicate and the more negative the child relates to siblings. Parental conflicts lead to the fact that the child has become an indicator of dominance within the family relation.