We examined 226 abdominal, retroperitoneal and sacrococcygeal tumours in newborns and infants aged 6 months or less. Most frequent were neuroblastomas (n = 83) followed by germ cell tumours (n = 76), 37 of which were immature and 32 were mature teratomas. Fully malignant germ cell tumours and malignant germ cell tumour components were much rarer in this age group (9.2%) than in older children (58.1%). The majority of germ cell tumours were localized in the sacrococcygeal region (72.4%). Next in frequency were tumours of the kidney (n = 54), including 28 congenital mesoblastic nephromas. In contrast to children over 6 months of age, kidney tumours were frequently low grade malignant with a favourable prognosis. Among liver tumours there were 19 infantile haemangioendotheliomas and 9 hepatoblastomas. By contrast, at ages 7-24 months hepatoblastomas (n = 28) were much more frequent than haemangioendotheliomas (n = 2). Tumours in the newborn and very young infant differ from those in older children in many respects: (1) the relative frequency; (2) localization; (3) distribution of histological types; (4) degree of differentiation, often associated with a tendency toward maturation but occasionally also with increasing malignancy; and (5) prognosis, which is comparatively favourable.