Warning others is a paradigm case of communicative helping and prospective action understanding. The current study addressed the ontogeny of warning in infants' gestural communication. We found that 12- and 18-month-olds ("n" = 84) spontaneously warned an adult by pointing out to her an aversive object hidden in her way ("problem" condition). In control conditions, the object was either positive ("no-problem" condition) or the adult had witnessed its placing ("problem-known" condition), which resulted in significantly less pointing. Results show that infants intervene spontaneously to help others avoid a problem before it has occurred. These acts of warning entail an understanding of negatively defined goals (to avoid an outcome) and incorrect representations of reality. Findings support theories of altruism and social-pragmatic competencies in infancy.