The frequency and types of chromosomal variability in regenerated Avena sativa L. plants were assessed by detailed meiotic analysis on 655 regenerated plants. Tissue cultures were initiated from immature embryos of the varieties Lodi and Tippecanoe and maintained by monthly subculturing. Plants were regenerated from 4-, 8-, 12-, 16- and 20- month-old cultures. Regenerated plants with cytogenetic alterations were common, although Lodi cultures produced a higher frequency of cytogenetically abnormal plants at each regeneration cycle than Tippecanoe cultures. After four months in culture, 49% of Lodi regenerated plants were cytogenetically abnormal, whereas only 12% of Tippecanoe regenerated plants were abnormal. The frequency of cytogenetically abnormal, regenerated plants increased with culture age. After 20 months in culture 88% of Lodi regenerated plants and 48% of Tippecanoe regenerated plants were cytogenetically abnormal. The most common cytogenetic alteration was chromosome breakage, followed by loss of a chromosome segment resulting in a heteromorphic pair at diakinesis. Of the regenerated plants classified as cytogenetically abnormal, 41% of Lodi plants and 66% of Tippecanoe plants had lost a portion of one or more chromosomes. Other alterations included trisomy, monosomy and interchanges. Chromosome instability associated with oat tissue cultures has several possible uses.