Following his training in internal medicine and hematology Dr. Barr joined the Faculty of Medicine at the University of Nairobi as part of an arrangement with the UK government to establish the first medical school in Kenya. While there he met Dr. Ian Magrath, then at the Lymphoma Treatment Centre in Kampala, Uganda. This was the beginning of a 3O-year friendship and professional association. Returning to Scotland, Dr. Barr joined the faculty of the University of Aberdeen before moving to the National Cancer Institute in Bethesda, MD as a visiting scientist.

Since 1977 he has been at McMaster University in Hamilton, Ontario where he is Professor of Pediatrics, Pathology and Medicine; and Chief of Service, Hematology-Oncology in the Childrenšs Hospital, Hamilton Health Sciences Corporation. Dr. Barršs main professional interests are international health, particularly cancer in childhood; late effects of cancer treatment, especially on nutritional status; and measurement of health-status and health-related quality of life (as a co-developer of the Health Utilities Index).

He is a member of the Treatment Committee of the International Union Against Cancer (UICC) and the committee for Pediatric Oncology in Developing Countries of the International Society for Pediatric Oncology (SlOP), as well as co-chair of the Canadian Childrenšs Cancer Surveillance and Control Program.