The symptom literature in cancer has primarily examined symptom severity, frequency and distress. Assessing cancer patients' perceptions of symptom importance-how important it is for them to see improvement in a symptom following an interventionand factors influencing these judgments would also inform patient-centred care, but this analysis has not been undertaken. This qualitative study aimed to identify factors underlying perceptions of symptom importance among 25 symptomatic metastatic breast cancer (MBC) patients. Participants were recruited from a cancer centre in the Midwestern USA. Semi-structured interviews focused on patients' rationale for considering common symptoms (i.e., anxiety, sadness, sleep problems, pain or fatigue) to be important. Thematic analyses revealed five interrelated factors underlying MBC patients' perceptions of symptom importance: activity restriction, concentration difficulties, exacerbation of other physical symptoms, symptom-related long-term health concerns and negative impact on their relationships with others. Patients most frequently stated that a physical or psychological symptom was important because of the resulting activity restriction. Additionally, some patients considered pain to be important because it signalled potential long-term health concerns, such as worsening metastatic disease. Findings suggest that clinicians should take into account MBC patients' perceptions of symptom importance and factors underlying these judgments when making shared treatment decisions. K E Y W O R D Sanxiety, fatigue, metastatic breast cancer, pain, psychological, symptoms