A girl, who was the product of the first recognized conception by healthy and chromosomally normal parents, and who was born 5 weeks before term with a birth weight of only 1,690 g, was found to have a ring-1 chromosome, r(1), in almost all cells analysed. A minor proportion of cells had a normal diploid chromosome complement including two regular No. 1 homologues. A few cells contained a large dicentric ring, and two monocentric ring-1 chromosomes were simultaneously observed in a few other cells. Q- and G- banding revealed the r(1) chromosome to be apparently non-deleted. The patient phenotypically resembled the three previously described human subjects with r(1) chromosomes, especially with respect to her remarkable growth failure and metal retardation. From consideration of data available on general development in subjects with an autosomal ring chromosome, it appears likely that the presence of such a ring chromosome per se in an early foetal chromosome complement may predispose to significantly retarded intrauterine and postnatal growth. Various genetical implications, including mitotic irregularities with subsequent loss of cells at division, are thought to account for the significant reduction of body mass in these patients.