Persistent diarrhea is prevalent in developing countries especially among lower socio-economic classes. Lactose intolerance is not a common cause of persistent diarrhea in infants and young children. This work was carried on 100 cases with persistent diarrhea (60 males and 40 females) randomly chosen among infants and young children attending pediatric out patients clinic of QENA General Hospital, LUXOR General Hospital, ASWAN General Hospital, ESNA General Hospital. Their ages were up to 3 years. The important finding in this work was that the negativity of stool analysis is not exclude the presence of lactose intolerance. Clinitest tablets was found to be more sensitive in detecting lactose intolerance