Erlotinib and gefitinib, two inhibitors of the epidermal growth factor receptor (EGFR), can stimulate apoptosis and differentiation of myeloid cell lines that lack EGFR, unveiling a novel, therapeutically exploitable off-target effect of tyrosine kinase inhibitors. Here, we performed a side-by-side comparison of erlotinib and gefitinib effects on a broad spectrum of malignant myeloid cell lines, as well as on primary myeloblasts freshly purified from the bone marrow of patients with myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML). Both erlotinib and gefitinib induce apoptosis of a cell line (KG-1) that represents AML, and differentiation in another cell line (P39) derived from a patient with high-risk MDS. In this setting, erlotinib was more efficient than gefitinib. Erlotinib and gefitinib were equipotent in inducing apoptosis of primary CD34+ myeloblasts from MDS and AML patients, yet had no toxic effect on CD34+ progenitor cells from healthy donors. Although the response of individual MDS and AML patients in vitro was highly heterogeneous, the pro-apoptotic effects of erlotinib and gefitinib correlated significantly. These results suggest that erlotinib and gefitinib share a mechanistically related off-target effect that may be taken advantage of for the therapy of MDS and AML.