Little is known of how the experience of Pediatric housestaff influences the decisions they make in the Neonatal Intensive Care Unit (NICU). We hypothesized that first year residents (interns) on-call in the NICU would order more tests per infant than more experienced residents. Our NICU has either an intern or a resident on-call by themselves at night, affording us a natural setting to compare intern vs. resident test ordering. We prospectively gathered data on all 785 infants admitted to our NICU in the academic year 1993-1994, cared for by 14 different interns and 17 residents. The outcomes of interest were number of tests ordered per infant by the on-call houseofficer: arterial blood gases (ABGs), x-rays, and electrolytes. Control variables gathered prospectively included the severity-of-illness of the individual infant, the on-call workload of the houseofficer (number of patients, admissions) and supervision (rounding frequency, on-call attending). Using multivariable linear regression, we found that interns ordered significantly more ABGs per infant than residents (p=0.02), amounting to some 0.33 ABGs per infant per call night (3.22 vs. 2.89 ABGs per infant) We found no difference between interns and residents in ordering x-rays or electrolytes. Assuming $54 per ABG and our average census of 9.5 NICU infants, interns ordered $175 more ABGs per call night than residents. We conclude that the experience of a houseofficer can impact on their decision to order an ABG on an infant in the NICU. Further research is required to see if the housestaff experience impacts upon a broader array of neonatal outcomes.