A hypermethylation defect associated with DNMT hyperactivity and DNMT3b overexpression characterizes a subset of breast cancers and breast cancer cell lines. We analyzed breast cancer cell lines for differential expression of regulatory miRs to determine if loss of miR-mediated post-transcriptional regulation of DNMT3b represents the molecular mechanism that governs the overexpression of DNMT3b that drives the hypermethylation defect in breast cancer. MicroRNAs (miRs) that regulate (miR-29a, miR-29b, miR-29c, miR-148a, miR-148b) or are predicted (miR-26a, miR-26b, miR-203, miR-222) to regulate DNMT3b were examined among 10 hypermethylator and 6 non-hypermethylator breast cancer cell lines. Hypermethylator cell lines express diminished levels of miR-29c, miR-148a, miR-148b, miR-26a, miR-26b, and miR-203 compared to non-hypermethylator cell lines. miR expression patterns correlate inversely with methylation-sensitive gene expression (r=-0.66, p=0.0056) and directly with the methylation status of these genes (r=0.72, p=0.002). To determine the mechanistic role of specific miRs in the dysregulation of DNMT3b among breast cancer cell lines, miR levels were modulated by transfection of pre-miR precursors for miR-148b, miR-26b, and miR-29c into hypermethylator cell lines (Hs578T, HCC1937, SUM185) and transfection of antagomirs directed against miR-148b, miR-26b, and miR-29c into non-hypermethylator cell lines (BT20, MDA-MB-415, MDA-MB-468). Antagomir-mediated knock-down of miR-148b, miR-29c, and miR-26b significantly increased DNMT3b mRNA in non-hypermethylator cell lines, and re-expression of miR-148b, miR-29c, and miR-26b following transfection of pre-miR precursors significantly reduced DNMT3b mRNA in hypermethylator cell lines. These findings strongly suggest that: i) post-transcriptional regulation of DNMT3b is combinatorial, ii) diminished expression of regulatory miRs contributes to DNMT3b overexpression, iii) re-expression of regulatory miRs reduces DNMT3b mRNA levels in hypermethylator breast cancer cell lines, and iv) down-regulation of regulatory miRs increases DNMT3b mRNA levels in non-hypermethylator breast cancer cell lines. In conlcusion, the molecular mechanism governing the DNMT3b-mediated hypermethylation defect in breast cancer cell lines involves the loss of post-transcriptional regulation of DNMT3b by regulatory miRs.