AIM: To determine the availability of donor hearts for children of different blood group and weight needing urgent heart transplantation. METHODS: Data maintained by the Australia and New Zealand Organ Donor Registry 1989-2004 were analysed to determine the frequency of donation. Probabilities of suitable donor availability within 10, 20, 30, 40, 60, 90 and 180 days were estimated using a Poisson model with the assumptions that traditional ABO blood compatibilities applied, suitable donors were 0.8-4.0 times the recipient's body weight (BW) and suitable adult donors were aged <40 years. RESULTS: Probabilities of suitable donor availability increase with passage of time from 10 to 180 days and decrease with competition from other needful recipients. Maximum suitable donor availability occurs for children of all blood groups at body weight 20 kg. The probabilities of a donor heart arising within 40 days (maximum safe duration of extracorporeal membrane oxygenation support locally available for young children) for this recipient body weight according to blood group is 0.89, 0.85, 0.73, 0.67 (AB, A, B, O). Probabilities for recipients of BW 3 kg and 60 kg respectively are 0.16, 0.14, 0.10, 0.09 (AB, A, B, O) and 0.66, 0.61, 0.47, 0.42 (AB, A, B, O). CONCLUSION: Expectation of suitable heart donation arising within 40 days for needful recipients in Australia is low for infants (probability <0.3), moderate for small children (probability 0.5-0.9) and modest for large children (probability 0.4-0.7), with variation at all body weights according to blood group and waiting time.