A role for microRNAs (miRNA) in human T-cell leukemia virus 1 (HTLV-1)–mediated cellular transformation has not been described. Here, we profiled miRNA expression in HTLV-1–transformed human T-cell lines and primary peripheral blood mononuclear cells from adult T-cell leukemia patients. Analyses of 11 different profiles revealed six miRNAs that were consistently up-regulated. Two of the up-regulated miRNAs (miR-93 and miR-130b) target the 3′ untranslated region (3′UTR) of the mRNA for a tumor suppressor protein, tumor protein 53–induced nuclear protein 1 (TP53INP1). A low expression level of TP53INP1 protein was found in HTLV-1–transformed cells. Additionally, when antagomirs were used to knock down miR-93 and miR-130b in these cells, the expression of TP53INP1 was increased, suggesting that the latter is regulated inside cells by the former. A role for TP53INP1 in regulating cell growth was established by experiments that showed that enhanced TP53INP1 expression increased apoptosis. Collectively, the findings implicate a miR-93/miR-130b-TP53INP1 axis that affects the proliferation and survival of HTLV-1–infected/transformed cells. [Cancer Res 2008;68(21):8976–85]