In the era of slowly increasing laboratory budgets, identifying usage patterns can be an important first step in decreasing unnecessary laboratory testing. From January 1, 1998, through December 27, 2002, we studied inpatient laboratory utilization at the University of Alberta Hospital, a large Canadian teaching hospital (665 inpatient beds with an almost 100% continuous occupancy rate). The daily numbers of complete blood cell count (CBC), CBC with differential (CBCD), international normalized ratio, activated partial thromboplastin time, vitamin B12, and red cell folate (FOL) tests were correlated with regularly occurring hospital events, such as weekends, statutory holidays, and resident changeover dates. Testing for hematological monitoring (CBC, CBCD) at our institution has been increasing at an average of 2% to 6% per year with a recent (2001 onward) rate of increase in hematological diagnostic testing (vitamin B12 and FOL) of close to 10%. As expected, laboratory usage was statistically significantly higher on weekdays compared with weekends. During the weekdays, ordering frequencies on Monday and Friday were shown to be statistically significantly higher than on the other days. A nadir occurred on Thursday and on the weekend. This finding was postulated to be due to a combination of attending physician unfamiliarity and defensive testing. Resident changeovers did not contribute to an increase in Monday laboratory testing, as was initially thought, nor did holidays. We believe that we can use our findings to decrease laboratory use. First, weekend or midweek use might become the appropriate standard for frequency of testing. Second, we must alert attending physicians (and residents who are soon to be attending physicians) about their subliminal need to increase testing during changeovers.