Background: Many countries across sub-Saharan Africa are rapidly increasing insecticide-treated net (ITN) coverage to combat malaria, but systematic data on the use of those ITNs and the factors affecting this use are scarce. Methods: A household survey was conducted during malaria season in 23 communities of Amhara and Oromia Regional States, Ethiopia, stratified by degree of urbanization (rural, peri-urban, or urban), whether or not they received indoor residual spraying (IRS), and whether or not free nets had been distributed. Descriptive statistics as well as univariate and multivariate logistic regression analyses were used to describe household net ownership and identify factors associated with use or non-use of nets already in the household. A qualitative component consisting of observations of ITNs in households and several open-ended questions provided further understanding of the reasons for ITN use and non-use. Results: Of 857 surveyed households, 91% owned at least one ITN, but only 65% of ITNs owned had been used the prior night. The multivariate analysis found that the factors significantly associated with an ITN being used were regional state (Amhara) (Odds Ratio [OR]=0.61; 95% Confidence Interval [C.I.] 0.43-0.86; p<0.01), residing in a house sprayed with IRS (OR=1.89; 95% C.I. 1.36-2.63; p<0.001), age of ITN (<12 months) (OR=0.55; 95% C.I. 0.37-0.82; p<0.01), shape (conical) (OR=2.27; 95% C.I. 1.10-4.68; p<0.05), and paying for the ITN rather than receiving it free (OR=2.16; 95% C.I. 1.32-3.53; p<0.01). The most common reasons for ITN non-use identified through the qualitative component of the study were: there are few mosquitoes around or malaria is not a serious problem; the ITN is no longer effective; ITN is in poor condition; the ITN is being saved. Observations showed many ITNs hanging incorrectly, and some being used for purposes other than as a bed net. Conclusions: The very high ITN ownership in the study areas suggests that a strategy targeting free nets to rural and poor households combined with support for the commercial sector is an effective means of achieving high coverage. The data suggests that use of ITNs owned could be increased by distribution of conical ITNs, continued development of the commercial sector, replacement schemes for worn-out ITNs, assistance with hanging of ITNs, and communication addressing misperceptions about malaria and ITNs.