Given the inherent complexity of rare paediatric diseases and the sensitive emotional context of the situations they create (due to the patients’ age and the tense uncertainty surrounding the progression of the disease), communication between the adults involved is a key tool in the efforts to provide these children and youths a better quality of life. We conducted ten interviews with families of children with rare diseases, in the aim of exploring how communication between doctors and patients affect their daily lives. All participants, members of FEDER (a Spanish federation of associations of patients with rare diseases) were invited by phone or email to participate in a semi-structured interview including questions on clinical information, communication experiences with healthcare professionals, and the impact these had on the interviewees’ relationships with them. To analyse these interviews, we used the ‘grounded theory’ methodology and open and axial text coding techniques, in addition to those identifying the properties and dimensions of the categories formulated. The core category we have proposed is ‘adjustment of mutual trust’, with said category describing the attitude and behaviour of doctors who inspire trust in the parents of paediatric patients diagnosed with a rare disease. More specifically, said behaviours or sources of trust are: appearing human, sensitive and empathetic; showing transparency and communicative openness; being supportive of parental proactivity; and being available to families at all times. Trust is the cornerstone of parent-doctor communication in the field of children with rare diseases. If the sources of trust are present, they create a degree of trust that bolsters both parties in the search for a common goal: providing the child with the best possible care.