BACKGROUND Under-nutrition the largest single underlying cause of death worldwide and is associated with over 1/3 of all childhood death and these children die before their fifth birthday. Under-nutrition contributes to the death of (5.6) million children death annually and its estimated 10 children die per minute. It is estimated that malnutrition, in particular under-nutrition, is the cause of more than half of all infant and child morbidity also 60% mortality in sub-Saharan Africa OBJECTIVES; To determine prevalence, risk factors and treatment outcomes of under-nutrition among children under five years old admitted at pediatrics department in KCMC from January 2014 to December 2015 METHODOLOGY; This study was a hospital based cross-sectional study. Under-nourished Children aged 0-59 months who admitted at KCMC pediatric department from 1st January 2014 to 31st December 2015 were included in the study. Data was extracted from the patient’s files available in medical records and prevalence was obtained from pediatric department using diagnostic registry book used during the study time. Data was recorded using the data extraction sheet. Collected was processed and analyzed with the aid of statistical package for the social science (SPSS) Version 16. Data consistency was checked and stored. Data was presented in frequency tables, charts and diagrams. Continuous variables were summarized into their measures of central tendencies and their respective measures of dispersion. P value of less than 0.05 with 95% confidence interval was used to estimate statistical significance of the association. RESULTS; out of 1528 admitted children, 186(12.2%) children were undernourished. 109(76.2%) were found to be severely undernourished. The risk factors associated with severe under-nutrition are co morbidities e.g. PAIDS and UTI with (p= 0.042, OR=0.1; CI= (0.002-0.9) and (p= 0.005, OR=4.1; CI= (1.5-11)) respectively. The recovery rate, mortality rate, average weight gain and hospital stay were of 73.4%, 26.6%, 0.7kg and 10 days respectively. High mortality rate and low weight gain in this study was due to co-morbidities especially PAIDS, anemia, meningitis, pneumonia, and septicemia also factors e.g. children aged <12 months and severe malnutrition were associated with death. CONCLUSION; Co-morbidities e.g. PAIDS, anemia, meningitis, pneumonia, and septicemia also factors e.g. low age and severe under-nutrition were associated with high mortality rate, poor weight gain and longer hospital stay.