Background: There are a subset of potentially modifiable co-morbidities that may be targeted in the preoperative phase with a view to optimizing control and improving post-operative outcomes. This study aims to estimate the effect of potentially modifiable co-morbidities on post-operative outcomes and to identify potential targets for preoperative management. Methods: Retrospective data on hospital separations in South Australia were analyzed using multiple regression to estimate the association between nine potentially modifiable co-morbidities and length of stay, post-operative complications and in-hospital mortality. Results: After adjusting for primary diagnosis, age, gender and other potential confounders, significant increases in length of stay and complications were recorded for eight and six of the nine modifiable co-morbidities, respectively. As examples, previous heart failure was associated with a 54% increase in length of stay and an odds ratio of 1.75 for complications. Asthma and chronic obstructive pulmonary disease was associated with a 38% increase in length of stay and an odds ratio of 1.64 for complications. Conclusions: A set of potentially modifiable co-morbidities is associated with a range of poorer post-operative outcomes, relative to patients without those co-morbidities. There is a clinical rationale that outcomes will be worse in the subset of patients for whom such co-morbidities are poorly controlled, and that timely intervention to improve control in the period prior to surgery will improve post-operative outcomes. Further research is required on post-operative outcomes for patients with and without controlled co-morbidities and on the effects of timely intervention to improve control prior to surgery.