Purpose: Clinical guidelines recommend partial nephrectomy (PN) as the preferred method of surgical excision of the small renal tumor whenever feasible. PN has comparable cancer cure rates to that of radical nephrectomy in this setting, and decreased risk of chronic kidney disease. A recognized devastating complication following partial nephrectomy is acute post-operative hemorrhage (APOH) from the reconstructed kidney. Risk factors for hemorrhage following partial nephrectomy remain poorly elucidated, as does the impact of hemorrhage on subsequent hospital stay. Identification of risk factors for hemorrhage may lead to a better understanding of and reduction of this complication. Material and Methods: We utilized a prospectively managed database comprised of patients undergoing open partial nephrectomy at our institution by the same surgical team from January 2006 to July 2012. Clinicopathologic factors assessed APOH for their relationships, including patient age, gender, diabetes, smoking, hypertension, coronary artery disease, American Society of Anesthesia Score (ASA), tumor size, RENAL nephrotomy score, pathologic result, cancer margin status, operative time, and intra-operative blood loss. The impact of APOH on subsequent hospital course was evaluated and compared with the entire cohort. Results: Data were analyzed from 200 consecutive patients. We identified 7 patients (3.5%) who experienced APOH. Compared with the entire cohort, APOH resulted in an increased hospital length of stay (median, 5 days; range, 2 -11 days, p = 0.001), an increased transfusion requirement (median, 6 units; range, 1 -16 units. p = 0.001), a greater risk of selective angiographic embolization (median, 2 procedures; range, 0 -3, p = 0.001), and completion nephrectomy (n = 2, p = 0.001). One patient in the APOH group experienced cardiac arrest and was resuscitated. Clinicopathologic factors associated with the increased risk of APOH in the present cohort were male gender (p = 0.03) and hypertension (p = 0.006). Conclusion: In the present analysis, APOH was associated with extended hospitalization, the increased transfusion requirement and the need for more ancillary procedures. APOH patients were at significantly increased risk of renal loss. Male gender and hypertension were associated with increased risk for APOH. We have incorporated this information into an APOH risk reduction program at our institution.