The aims of this study were to describe the development of sequelae in obstetric brachial plexus palsy (OBPP) and to identify possible differences in functional outcome from 5 years of age to follow-up, 2 to 15 years later. A cohort of 70 participants (35 males, 35 females; age range 7-20y, mean 13y 6mo [SD 4y 3mo], median 13y) with OBPP of varying degrees of severity were monitored. Differences in status between 5 years of age and follow-up were studied. Active joint motion in the shoulder and hand function, especially grip strength, generally remained unchanged or improved, whereas a slight but significant deterioration occurred in elbow function. Shoulder surgery resulted in considerable improvement of shoulder function. Participants with nerve reconstruction had a similar profile of change as the non-operated group. It was concluded that ongoing follow-up of children with OBPP, beyond the preschool years, is required due to decreases in elbow function, a commonly occurring restriction in external rotation of the shoulder, together with individual variations in long-term outcomes. In a related article (part II: neurophysiological aspects) long-term neurophysiological and sensory aspects of OBPP are reported.