Infantile pyknocytosis(IP) is a rare, self-limited neonatal haemolytic anaemia that may require multiple blood transfusions. Only a little more than 50 cases have been reported in the medical literature, and the great majority of them concerns term infants. The etiology of IP is not well understood; most likely it results from a transient extra-corpuscular factor, whose nature is unknown, transmitted from mother to child or, alternatively, from a deficiency of an anti-oxidative agent. We report the case of two preterm twins, one of which suffered from IP and developed severe anaemia at age 2 wk, while the other was unaffected. Although no specific agent was identified as the cause of anaemia and IP, we speculate that the transmission of an agent from mother to child was unlikely, as only twin one suffered from IP. Smelly greenish diarrhoea occurred just before the presentation of IP, suggesting that the same agent led to both the diarrhoea and the oxidative injury. Because IP may remain underdiagnosed, it should be considered in cases of early unexplained severe hemolytic anemia. Infantile pyknocytosis (IP) is a rare, self-limited neonatal haemolytic anaemia that may require multiple blood transfusions. Only a little more than 50 cases have been reported in the medical literature, and the great majority of them concerning term infants. The etiology of IP is not well understood; most likely it results from a transient extra-corpuscular factor, whose nature is unknown, transmitted from mother to child or, alternatively, from a deficiency of an anti-oxidative agent. We report the case of two preterm twins , one of which suffered from IP and developed severe anaemia at age 2 wk, while the other was unaffected. Although no specific agent was identified as the cause of an anemia and IP, we speculate that the transmission of an agent from mother to child was unlikely , as only twin one suffered from IP. Smelly greenish diarrhoea occurred just before the presentation of IP, suggesting that the same agent led to both the diarrhea and the oxidative injury. Because IP may remai n underdiagnosed, it should be considered in cases of early unexplained severe hemolytic anemia.