The ethanolic extract of a Malagasy species Euphorbia stenoclada (ES) (Euphorbiaceae), traditionally used as a herbal remedy against asthma and acute bronchitis, was tested to evaluate possible anti-proliferative activity on human airway smooth muscle cells (HASMC). The ES ethanolic extract totally abolished the interleukin-1 (IL-1) induced proliferation of HASMC (IC50 = 0.73 ± 0.08g/mL). No cytotoxic effect was observed up to 20 g/mL. A bioassay-guided fractionation of the ethanolic extract was performed by reversed-phase (RP) flash chromatography, giving five fractions (FA to FE) where fraction FE was the only active one (IC50 = 0.38 ± 0.02g/mL). The purification of this bioactive fraction FE was carried out by RP-HPLC affording six sub-fractions 1–6, and only sub-fraction 5 kept the anti-proliferative activity. Its major constituent was identified as quercetin (IC50 = 0.49 ± 0.12g/mL) by means of HPLC/UV/MS and co-elution with the authentic standard. Quercitrin was also identified in the fraction FE but was inactive. A structure–activity relationship with flavonols determined that methylation reduced the anti-proliferative activity whereas glycosylation abolished it. The present study shows that the anti-proliferative properties of Euphorbia stenoclada are mediated through the presence of quercetin that may explain the traditional use of this plant as a remedy against asthma. © 20