The aim of the present study was to evaluate whether the application of Dutch versus US test procedures and norms of the Bayley Scales of Infant Development - 2nd edition(BSID-II) leads to different developmental outcomes. The BSID-II was administered to 376 preterm infants (191 males, 185 females; mean gestational age 30wks [SD 2.7], mean birth weight 1242g [SD 385]) at corrected ages of 6,12,24, and/or 36 months. Raw scores were calculated twice with US and Dutch test procedures. Raw scores as well as Mental Developmental Index (MDI) and Psychomotor Developmental Index (PDI) scores, calculated on the basis of Dutch versus US normative data, were compared. Small but statistically significant Dutch-US differences were found between raw scores. Large, clinically relevant Dutch-US differences were found for MDI and PDI scores, especially at 6 and 12 months. These differences were likely to have been caused by a bias in the Dutch normative data, although a slower developmental pace of Dutch children in general could also have a role. This study highlights the problems that can occur when using a test that was developed in another country, even when local standardization is available.