Since a small metacentric marker chromosome, probably an isochromosome for the short arm of chromosome 5, was found in about three-quarters of carcinomas of the cervix in recent studies using chromosome banding techniques, 36 tumours from the prebanding era which yielded excellent chromosome spreads were reviewed: it was found that a similar proportion of these tumours had a small metacentric marker, on average 38% longer than the chromosomes 19 and 20, often in two copies. The marker was also seen in one of two cervical carcinomas in situ. This study on archival material lends support to the view that the marker represents a characteristic and significant finding in this class of tumour.