Background: Peyronie's disease (PD) can be subdivided into acute and chronic phases. Intralesional collagenase Clostridium histolyticum has been shown to improve curvature in the chronic phase. Initial clinical trials excluded patients in the acute phase from treatment. Recent studies show comparable results among men in the acute phase. The definition of acute phase varies among existing studies, but it is generally understood to last 12-18 months and is accompanied by penile pain and progression of deformity. We sought to evaluate the safety and efficacy of intralesional collagenase injection therapy during the acute phase of PD using multiple definitions of the acute phase. Methods: All men receiving intralesional collagenase for PD from October 2015 through December 2020 at a single academic institution were retrospectively assessed for patient demographics and comorbidities, pre- and post-treatment curvature, and adverse events. Two definitions of acute phase were used: (I) acute phase duration ≤6 months, chronic phase duration >6 months; and (II) acute phase duration ≤12 months with penile pain, chronic phase duration >12 or no penile pain. Results: Of 330 patients identified, 229 underwent intralesional collagenase treatment with pre- and post-treatment erect penile goniometry. 65 (28%) met criteria for definition 1 of acute phase, 37 (16%) met criteria for definition 2, and 76 (33%) met criteria for either. Percent change in penile curvature was not significantly different between acute and chronic phases using definition 1 (16.0% vs. 16.6%, P=0.89), definition 2 (19.9% vs. 15.7%, P=0.43), or either (16.5% vs. 16.3%, P=0.96). The rates of development of bruising, swelling, hematoma, or corporal rupture were not significantly different between the acute and chronic phases under either definition (all P>0.05). Conclusions: This single-center, retrospective cohort analysis suggests that intralesional collagenase is both safe and effective for the treatment of men with acute phase PD. Limitations exist inherent to retrospective review, since many men did not return for post-treatment goniometry, possibly skewing our cohort toward incomplete responders. Prospective, randomized studies will be required to confirm these findings.