To study the impact of chromosomal abnormalities on the clinical behavior of testicular nonseminomatous germ cell tumors (TNSGCTs), we compared the chromosomal constitution of primary tumors of patients who initially presented and remained without metastases to those with metastatic disease. Furthermore, the chromosomal pattern of primary TNSGCTs was compared to ploidy and the clinicopathologic risk factors histology and small-vessel invasion. The modal chromosome number and the ploidy were in agreement. No correlation was found between the modal chromosome number and histology, presence of vascular invasion, or clinical stage. No correlation was found between structural chromosome abnormalities, like the number of copies of the i(12p) chromosome, and clinical stage. No obvious differences were found in chromosomal constitution of metastatic and non-metastatic tumors. The results of the present study suggest that in TNSGCTs differences in clinical behavior are not associated with gross chromosomal differences.