Breast implant-associated anaplastic large cell lymphoma (BIA-ALCL) is a very rare type of T-cell lymphoma, uniquely caused by a single environmental stimulus. Here we present a comprehensive genetic analysis of a relatively large series of BIA-ALCL (n=29), for which genome-wide chromosomal copy number aberrations (CNA) and mutational profiles for a subset (n=7) were determined. For comparison, CNAs for ALK-negative nodal-ALCLs (n=24) were obtained. CNAs were detected in 94% of BIA-ALCLs with losses at chromosome 20q13.13 in 66% of the samples. Loss of 20q13.13 is characteristic for BIA-ALCL as compared to other classes of ALCL, such as primary cutaneous ALCL, systemic type ALK-positive and -negative ALCL. Mutational patterns confirm that the IL6-JAK1-STAT3 pathway is deregulated. Although this is commonly observed across various types of T-cell lymphomas, the extent of deregulation however is significantly higher in BIA-ALCL as indicated by pSTAT3 immunohistochemistry. The characteristic loss of chromosome 20 in BIA-ALCL provides further justification to recognize BIA-ALCL as a separate disease entity. Moreover, CNA analysis may serve as a parameter for future diagnostic assays for women with breast implants to distinguish seroma caused by BIA-ALCL from other causes of seroma accumulation such as infection or trauma. Copyright © 2020 American Society of Hematology.