Discrepancies between patient wishes and end-of-life treatment decisions have been documented, and the determinants of end-of-life treatment decisions are not well understood. Our objective was to understand hospital staff perceptions of the role of acute care hospital medical doctors in end-of-life treatment intensity. In 11 purposively sampled Pennsylvania hospitals, we completed 108 audiotaped semistructured interviews with key informants involved in decision making or discharge planning. Using grounded theory, we qualitatively analysed transcripts using constant comparison to identify factors affecting end-of-life treatment decisions. A predominant theme identified was that end-of-life treatment intensity depends on the doctor. Communication with patients and families and collaboration with other care team members also were reported to vary, contributing to treatment variation. Informants attributed physician variation to individual beliefs and attitudes regarding the end-of-life (religion and culture, determination of when a patient is dying, quality-of-life determination and fear of failing) and to socialization by and interaction with the healthcare system (training, role perception, experience and response to incentives). When end-of-life treatment depends on the doctor, patient and family preferences may be neglected. Targeted interventions may reduce variability and align end-of-life treatment with patient wishes.