Lactobacillus rhamnosus (573L/1-3) strains are considered effective in the treatment of rotaviral diarrhoea in children. The colonisation of the gastrointestinal (G.I.) tract by the Lactobacillus rhamnosus strains and the determining factors are discussed reporting data of a prospective, double-blinded, placebo-controlled randomized study in children between the 2nd month and 6th year of life with acute diarrhoea lasting not longer than 5 days. The examined strains were detected in 37/46 (80.43%) patients after 5 days and in 19/46 (41.3%) patients after 14 days since the start of the treatment. L. rhamnosus 573L/1 strain colonised the G.I. tract more persistently. L. rhamnosus strains are effective in colonising the G.I. tract during acute diarrhoea. Persistence of colonisation is dependent on the properties of administered probiotic strains.