['Breastfeeding performance index is an explanatory attempt to summarize key breastfeeding practices by summarizing the different dimensions of breastfeeding practices into a single summary variable. Breastfeeding performance index is used to assess optimal breastfeeding practices by constructing a single composite index that includes timely initiation of breastfeeding, prelacteal feeding, current breastfeeding status, bottle feeding, any liquid given(except medicine) in the last 24\xa0h, formula given in the last 24\xa0h, any solid food given in the last 24\xa0h. This study aimed to assess optimal breastfeeding practices of 0-6 month infants using breastfeeding performance index (BPI) and its association with childhood illness in Ethiopia.', 'A secondary data analysis was carried out based on the Ethiopia Demographic and Health Survey (EDHS) 2011 data. The BPI was created using seven components by giving equal weight for all components during scoring. The data were described using descriptive statistics and analyzed using multivariable logistic regression.', 'The prevalence of low, medium, and high BPI was 18.41, 57.96 and 23.63\xa0% respectively. The mean BPI score was 4.38 (SD 1.25; 95\xa0% CI 4.31, 4.45). Multivariable logistic regression analysis showed that those infants who had low BPI score were 2.22 times (AOR\u2009=\u20092.22; 95\xa0% CI 1.20, 4.11) and medium BPI category had 2.15 times at higher odds (AOR\u2009=\u20092.15; 95\xa0% CI 1.23, 3.75) of developing diarrhea compared to infants in the highest BPI category. Being in the lower BPI category was significantly associated with higher odds of having fever (AOR\u2009=\u20091.73; 95\xa0% CI 1.06, 2.80). Being in the medium index category was also associated with higher odds of having short and rapid breaths (AOR\u2009=\u20092.02; 95\xa0% CI 1.01, 4.04).'] Is higher breastfeeding performance index associated with lower risk of illness in infants under six months in Ethiopia?