We examined whether lenalidomide exposure up-regulates miRNAs and mRNAs, previously shown to play a role in the disease phenotype of del(5q) myelodysplastic syndrome, in pre-treatment CD34(+) marrow cells. We hypothesized that increased expression would predict for clinical response. Changes in miR-143, miR-145, miR-146a, miR-146b, miR-378, miR-584, SPARC and RPS14 were examined in del(5q) (n=10) and non-del(5q) (n=18) myelodysplastic syndrome patient samples. Significantly increased expression of miR-143 (1.8-fold and 1.5-fold in del(5q) and non-del(5q), respectively), and miR-145 (1.9-fold and 1.6-fold in del(5q) and non-del(5q), respectively) was observed. In the del(5q) myelodysplastic syndrome cohort, transfusion independence correlated with a 1.3-fold or more increase in miR-145 expression and response over 12 months correlated with a 1.5-fold or more increase. Knockdown of miR-143 and miR-145 in cord blood CD34(+) cells resulted in increased erythroid progenitor activity. Lenalidomide selectively abrogated progenitor activity in cells depleted of miR-143 and miR-145 supporting a key role for miR-143/145 in the sensitivity to lenalidomide of del(5q) myelodysplastic syndrome patients.