Horner syndrome may be seen in infants with extended Erb obstetric brachial plexus palsy. However, its prognostic value in these infants has not been previously investigated. A total of 220 infants with extended Erb palsy were included and divided into 2 groups: group I (n = 209) were infants with extended Erb palsy without Horner syndrome, and group II (n = 11) were infants with extended Erb palsy and concurrent Horner syndrome. The rate of good spontaneous recovery of elbow flexion was 59% in group I and 27% in group II, and the difference was significant (P = .038). The rate of good spontaneous recovery of wrist extension was 61% in group I and 0% in group II, and the difference as highly significant (P < .0001). Concurrent Horner syndrome in infants with extended Erb palsy may be considered as a poor prognostic sign for recovery of the sixth and seventh cervical roots.