A study was made of several epidemiologic features of 1,134 sudden, unexpected infant deaths which occurred in Cuyahoga County (Cleveland), Ohio during the decade 1956 through 1965. The frequency of occurrence of these deaths was greater among nonwhites (5.85 per 1,000 live births) than among whites (2.39 per 1,000 live births) and higher among male infants (3.50 per 1,000 live births) than among female (2.70 per 1,000 live births). Grouping of sudden, infant deaths with respect to median income of the census tract of residence of the child showed progressively lower rates for both whites and nonwhites with increasing income. Three fourths of the deaths occurred in infants less than 25 weeks of age. Sudden, unexpected infant deaths fluctuated by season, with greater frequency in cold months than in warm months.