The rate of spontaneous and methotrexate (MTX)-induced chromosome breakage was studied in individuals with a history of habitual abortions in whom a structural chromosomal aberration was found in a single cell during routine cytogenetic analysis. Twelve such individuals were selected because they were not under the influence of any known mutagenic factor such as smoking, alcohol, medication and apparent irradiation; they were compared to 12 age- and sex-matched control parents. A detailed statistical analysis revealed that the spontaneous and MTX-induced chromosome breakage was significantly increased in the abortion group. The MTX-induced breakage rate was especially elevated in the women of the abortion group.