Objective To evaluate phototherapy practices by measuring the irradiance levels of phototherapy (PT) devices. Design Prospective study. Setting Tertiary neonatal intensive care units. Patients None. Interventions Irradiance levels of PT devices used in the 10 Dutch Neonatal Intensive Care Units (NICUs) were measured according to the local PT practice patterns. The irradiance levels of all overhead and fibre-optic PT devices were measured with a radiometer using an infant silhouette model. Results Eight different PT devices were used in the 10 NICUs; five were overhead devices and three fibre-optic pads. The median (range) irradiance level for overhead PT devices was 9.7 (4.3–32.6) µW/cm2/nm and for fibre-optic pads 6.8 (0.8–15.6) µW/cm2/nm. Approximately 50% of PT devices failed to meet the minimal recommended irradiance level of 10 µW/cm2/nm. Maximal irradiance levels for overhead PT spot lights were inversely related to the distance between device and infant model (R2=0.33). The distances ranged from 37 cm to 65 cm. Conclusions PT devices in the Dutch NICUs show considerable variability with often too low irradiance levels. These results indicate that suboptimal PT is frequently applied and may even be ineffective towards reducing total serum bilirubin levels. These results underline the need for greater awareness among all healthcare workers towards the requirements for effective PT including measurements of irradiance and distance.