Background: Malnutrition is a major health problem, especially in developing countries. It affects almost 800 million people. Incidence rates vary among different continents of the world. PEM is undoubtedly the most serious nutritional problem affecting several thousand young children in Egypt. This study was aimed to find out the epidemiology of under- nutrition and PEM among children less than 5 years old. Patients and methods: a cross sectional randomized study was carried out on 1026 children of age group from one month to 5 years old. Ethical clearance was obtained and permission from respective authorities was also taken. The instrument used is a randomized method (simple random sample) from MCH centers and OPD of Al- Azhar University hospital in Assiut. History taking, physical examinations including nutritional assessment clinically and anthropometrically (using WHO growth charts) was done. Data was collected and statistically analyzed and represented in tables and figures. Results: we found that age of 17.1 % of studied children was less than one year. 21.4% aged 1 -2 years and 61.5% of children aged 2 - 5 years, 54.7 % were females, 63.9. % were from rural areas and 54.5 % were from low social class families.. The overall prevalence of under-nutrition was 35.5 %. The trader-nutrition was more frequent among children less than 2 years of age (65.9 %), (p value < 0.001). Females were more suffering from under-nutrition (57.4 %), (p value 0.0491). More than 70 % of under-nourished children were from rural areas (p value 0.002). Children of first order of birth were more suffering from under-nutrition followed by those of 3r and fourth order of birth. More than 80 % of under- nourished children were from low social class families (p value < 0.001). in our study we found a statistically significant differences between well and under-nourished children according to some clinical parameters (feeding, history of infections, hospital admissions, vaccinations and error in weaning), (p value < 0.0001), where under--nutrition was frequent among artificially fed children and those with error in weaning, infections especially diarrheal diseases and frequent hospital admissions were major -risk factors for under- nutrition. More than 58 % of under-nourished children were simple under- weight, 24 % stunted, 10.46 % wasted and 4.9 % wasted and stunted. Incidence of simple under-weight (58.8) was more than % of Marasmus (26.65 %). Kwashiorkor constitutes 11.82 % while incidence of Marasmic kwashiorkor was 2.74 %. A statistically significant differences between children of different age groups as regard incidence of types of under-nutrition, where simple under-weight was more prevalent among children aged 1-2 years old followed by children aged 2-5 years (p 0.021). Stunting (p value 0.452) and wasting (p value 0.025) was more prevalent among children less than one year of age, while combined wasting and stunting were more incidence among children aged 1-2 years old (p value 0.031). The incidence of Marasmus, Kwashiorkor and Marasmus Kwashiorkor were more incidences among children less than one year old, while simple under-weight more prevalent among children aged 2-5 years. A statistically significant difference between both sex as regard types of PEM, where simple under-weight and Marasmus Kwashiorkor were more prevalent among females, while Marasmus and Kwashiorkor were more prevalent among males. The overall incidence of under-nutrition among children from rural areas was 39% compared to those from urban areas (29.5 %). Marasmus, kwashiorkor and Marasmus kwashiorkor were more prevalent among children from rural areas. A statistical non-significant difference was found between children from urban and rural areas as regard incidence of simple under-weight (p value > 0.05) Keywords: incidence, under-nutrition, PEM, marasmus, kwashiorkor.