Attention is drawn to the marked change in the pattern of infant mortality in Canada. The period of greatest improvement in the reduction of infant death, i.e. from the end of the first 28 days of life to the end of the first year, continues to present a challenge in the problem of sudden unexpected deaths (S.U.D.). Cases remain unexplained even after detailed autopsy examination. The principal theories of milk hypersensitivity and virus infection have been unsatisfactory, to date, in accounting for the deaths.By enlisting the co-operation of parents and physicians, the Department of Preventive Medicine at Queen's University plans to carry out an epidemiological investigation of S.U.D. in infancy in Southeastern Ontario. Much could be added to our knowledge of S.U.D. by the examination of the sociomedical background of cases.