Centrosome amplification (CA) contributes to carcinogenesis by generating aneuploidy. Elevated frequencies of CA in most benign breast lesions and primary tumors suggest a causative role for CA in breast cancers. Clearly, identifying which and how altered signal transduction pathways contribute to CA is crucial to breast cancer control. Although a causative and cooperative role for c-Myc and Ras in mammary tumorigenesis is well documented, their ability to generate CA during mammary tumor initiation remains unexplored. To answer that question, K-Ras(G12D) and c-Myc were induced in mouse mammary glands. Although CA was observed in mammary tumors initiated by c-Myc or K-Ras(G12D), it was detected only in premalignant mammary lesions expressing K-Ras(G12D). CA, both in vivo and in vitro, was associated with increased expression of the centrosome-regulatory proteins, cyclin D1 and Nek2. Abolishing the expression of cyclin D1, Cdk4 or Nek2 in MCF10A human mammary epithelial cells expressing H-Ras(G12V) abrogated Ras-induced CA, whereas silencing cyclin E1 or B2 had no effect. Thus, we conclude that CA precedes mammary tumorigenesis, and interfering with centrosome-regulatory targets suppresses CA.