During the years 1987-89, transient erythroblastopenia of childhood was diagnosed in 52 previously healthy Swedish children aged less than 4 y. Among these children there were four pairs of siblings, including one pair of identical female twins. This is a much higher familial occurrence than expected. The probability of finding 4 pairs of siblings with this disease in 50 families was estimated to be considerably less than 10-6. In the retrospectively analysed material, no environmental factor was implicated and no association with human leucocyte antigen could be proven. The twins demonstrated the disease simultaneously. Their anaemia was transient and did not recur, but showed certain features usually seen in congenital hypoplastic anaemia. The other pairs of siblings fulfilled the criteria for transient erythroblastopenia of childhood and several years elapsed between the development of the disease in siblings. Two of the fathers were reported to have had transient anaemia during their childhood. Our findings indicate that transient erythroblastopenia of childhood may involve hereditary factors, eventually demonstrating an autosomal dominant inheritance.