Children with sickle cell anaemia (SCA) and conditional transcranial Doppler (TCD) [time-averaged mean velocity (TAMV) 170-199 cm/s] have increased risk of primary stroke, but receive no specific therapy. Some will convert to abnormal velocities (TAMV >/=200 cm/s) with further increase in stroke risk. In 2003, our centre initiated universal TCD screening, targeting all children (aged 2-16 years) with SCA. TCD examinations were repeated at intervals based on initial results. To determine rates and risk factors for TCD conversion, we reviewed all examinations since 2003, excluding patients receiving hydroxycarbamide (hydroxyurea) or transfusions. Of the eligible population, 274 children (98%) were screened at a median age of 7.1 years (range 1.7-18.2). Fifty-four patients (20%) had conditional TAMV either on initial screening or a subsequent examination. The 18-month cumulative incidence of conversion from conditional to abnormal TAMV was 23%. Age, initial TAMV, laboratory values, blood pressure and oxygen saturation were not significantly associated with conversion. Our cohort provides systematic longitudinal evaluation of an unselected paediatric population universally screened and retested at regular intervals. Our data document a high conversion rate to abnormal velocities among untreated children with SCA. Therapy should be considered for the prevention of conversion to abnormal TCD velocities.