Evidence-based guidelines recommend few routine investigations for healthy adults at the periodic health examination (PHE). However, small studies indicate that laboratory tests are commonly ordered at the PHE. This study examined PHE laboratory testing that is not recommended by recognized guidelines ('discretionary'). Using administrative data from the universal health care system in Ontario, Canada, we studied 792,844 adults having a PHE in 1996 and the 3,727 physicians who administered them. We measured the number of discretionary laboratory tests per PHE along with the patient and physician factors potentially influencing laboratory testing. A multilevel, multivariate model was used to examine the association between the number of discretionary laboratory tests at the PHE with patient and physician characteristics. A mean of 7.1 discretionary tests (SD 7.1) was ordered per PHE. Renal, haematological, glucose and lipid tests each were conducted in more than a third of PHEs. Testing varied extensively between physicians and was more common in healthy patients. With the exception of age, patient factors had little effect on discretionary testing. However, each physician factor we examined was independently associated with the number of discretionary tests. Physician specialty, practice volume and previous testing patterns had the strongest influence on discretionary testing. Discretionary investigations are common at the PHE. Testing varies extensively between physicians and seems to be driven more by physician than by patient factors. Interventions to modify discretionary test utilization at the PHE should consider these physician factors.