Friendships are significant to child development and health but diseases such as cancer can interrupt the contact with friends. The purpose of this study was to describe perceptions of friendship from the perspective of children undergoing cancer treatment, in order to build knowledge that can be used in a health promotion intervention for these children. Fifteen children between 8 and 12 years of age participated in focus groups, where a mixture of informative and creative techniques were used. The focus group discussions were analyzed using qualitative content analysis. The analysis resulted in three generic categories, Common interests and experiences, Mutual empathic actions. and Mutual trust and understanding, incorporating seven subcategories. Based on children's descriptions from a salutogenic perspective, friendship emerged as An equal and mutual commitment that evolves over time and with interactions face-to-face and digitally, a child perspective on friendship should be central to the development of health promotion interventions designed to support friendship relations of children treated for cancer.