The aim of the study was to determine the differences in upper limb function and activity/participation levels between preschool children with Narakas Groups 2a and 2b obstetric brachial plexus injury; and to determine the significance level of the factors affecting upper limb functions in these patients. Sixty-seven children, aged 3 to 7, who had not had surgical intervention, were evaluated in terms of joint movements, modified Mallet classification, Raimondi hand classification, brachial plexus outcome measure, paediatric outcome data collection instrument and stereognosis. There were significant functional differences between the groups, in favour of Group 2a. The movements affecting total function of the upper limb were: hand to spine (p < 0.001), global abduction (p < 0.001) and hand to mouth (p < 0.001), in descending order of significance. Passive internal rotation was the most important passive joint movement affecting shoulder function (p < 0.001). The results of this study suggest that more emphasis should be placed on the shoulder internal rotation in treatment strategies.