BACKGROUND Hypertension (HT) is a common and serious complication following renal transplantation in children, and an important risk factor for cardiovascular morbidity and mortality. This study evaluated the clinical characteristics of HT in children after renal transplantation. METHODS Twenty-four children who were followed up at least 6 months after renal transplantation were enrolled in the study. From the clinical records, demographic and laboratory data, casual blood pressure (BP) measurement, ambulatory BP monitoring (ABPM), medication, and left ventricular mass index (LVMI) at echocardiogram were documented. RESULTS Mean age at time of transplantation was 12.6 ± 3.0 years and mean follow-up period was 19.6 ± 15.8 months. HT was detected in 21 children (87.5%) after renal transplantation. Twelve patients (50%) had HT both before and after transplantation and nine (38%) had HT only after transplantation. HT developed in 67% within the first week and in 95% within the first month. All hypertensive children had night-time HT and no child had isolated daytime HT. The efficacy of HT control was 42%. Median LVMI in patients with HT after renal transplantation was 42.3 g/m(2.7). CONCLUSIONS Severe HT, an important complication, was frequently seen in the early period after renal transplantation. Predominance of nocturnal HT and the lack of isolated daytime HT after transplantation underline the importance of ABPM. ABPM should be performed regularly in the first year after transplantation, not only for diagnosis but also for evaluation of HT control.