OBJECTIVE There is a limited amount of studies with results on professional development of paediatric oncology nurses. This study seeks to increase the understanding of the factors associated with the professional development of paediatric oncology nurses through the continuous education programme from the paediatric nurses' perspective. METHODS The descriptive, qualitative study used the text of participants' electronic diaries as data during a two-year continuing professional education programme in 2016-2018. The sample consisted of 17 paediatric oncology nurses who were working in three different university hospitals. The data were analysed with the inductive content analysis method. RESULTS Professional development is linked with a strong knowledge base in nursing, which involves the use of nursing methods and up-to-date nursing practices. Professional development is also linked with the use of medical knowledge, which manifests as a deep understanding of cancers and their treatment. CONCLUSIONS Research results show that a strong knowledge base in nursing alone is not sufficient for the professional development of paediatric oncology nurses. They also need to use their medical knowledge in order to gain an adequately deep understanding of children's cancers and their treatment. Nursing must be organised so that nurses have the opportunity to compare, share, question and argue for the methods they use with their colleagues in their own unit and other hospitals. Further research is needed on the professional development of paediatric oncology nurses and factors affecting it in order for a career development model to be created for this specific yet demanding area of nursing.