Background There has been little formal study of blood pressure in children after cardiac transplantation. Methods Twenty-four-hour and clinical blood pressure (BP) were measured in 28 children (>6 months after transplantation) and compared with a large amount of normal data. Results Conventional (clinical) systolic BP (SBP) was elevated in 9 (32.1%) of 28 (95% confidence interval [CI] 15.8 to 52.3), and conventional diastolic BP (DBP) was elevated in 5 (17.8%) of 28 (95% CI 6.0 to 36.8). Mean 24-hour BP was >97.5 percentile in 2 (7.7%) of 26 (95% CI 0.9 to 25.1) for SBP and in 7 (28.0%) of 25 (95% CI 12.1 to 49.4) for DBP. In comparison with the control population, mean nighttime SBP was 8.9 mm Hg higher in the transplanted group (95% CI 4.8 to 13.1), but daytime and mean 24-hour SBP were similar. Mean day, night, and 24-hour DBP was significantly higher in the transplanted patients. The nighttime decrease in BP was significantly less than controls for SBP, but not for DBP. Conventional BP measurement was poorly predictive of 24-hour BP. There was a significant association between mean 24-hour SBP and interventricular septal thickness ( r 2 = 0.35; p = 0.01). DBP was not associated with interventricular septal thickness ( r 2 = 0.07; p = 0.20) but was significantly correlated with the time since transplantation ( r = 0.42; p = 0.03 for conventional DBP and r = 0.43; p = 0.04 for 24-hour DBP). Conclusions The elevation of DBP in children after cardiac transplantation is unexplained. The elevation in nighttime SBP has possible important therapeutic implications and is not predicted by conventional (clinical) BP measurement.