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An analysis of sex ratio was made for chromosomally normal and trisomic spontaneous abortions identified in a cytogenetic survey of spontaneous abortions. For the chromosomally normal group, the extent of maternal contamination among 46, XX abortions was determined by comparing chromosome heteromorphisms of the parents with those from the tissue samples. The sex ratio among the normal abortions was then estimated to be approximately 1.30, after correcting for the maternal contaminants and 46, XX hydatidiform moles of androgenetic origin. This estimate is significantly higher than values typically reported for newborns, indicating an effect of X-linked genes acting in utero. The sex ratio among trisomic abortions identified in the present study and in four other cytogenetic studies of spontaneous abortions was also significantly greater than 1.0, but not as high as the estimates of sex ratio for the chromosomally normal abortions. There was considerable variability among individual trisomies, and possible mechanisms leading to this variation are discussed.