OBJECTIVE: To study tumor progression in a simulation model following tetraploidization (TPZ) and the establishment of aneuploid tumors in a population of young women (aged < 40 years) with breast cancer. STUDY DESIGN: A sample of 284 women aged < 40 years, retrieved from 5,875 randomized breast cancer patients treated from 1991-2000, were studied. The tumors were analyzed for chromosomal instability and proliferative activity called the stem-line scatter index (SSI). The dynamic interactive relationship between diploid, tetraploid, and aneuploid tumors was followed during increasing age and related to SSI values and tumor size. RESULTS: TPZ occurred early in tumor progression in the lower ages and was followed by hypotetraploid and further down to hypertriploid tumors, in the end reaching hypotriploid tumors for women aged >34 years. During tumor progression we observed a reduced frequency of diploid tumors from 71.7% to 14.3% (p < 0.00001), and triploid tumors increased from 13.2% (6 < SSI < 15 relative units) to 40.2% (20