Background: The risk factors determining the aggressiveness of papillary thyroid microcarcinoma (PTMC) are not well known. Aim: To determine if tumor size, along with other features of the tumor, influences its prognosis. Patients and Methods: We analyzed the medical records of 147 patients (age range 16-92 years, 93% women) at the Clinical Hospital of University of Chile who underwent thyroid surgery and in whom at least one focus of PTMC was found. We determined the association between different clinical characteristics and the presence of capsular invasion,. lymph nodal extension or recurrence. Results: A tumor size over 5 mm, a follicular subtype and being aged more than 45 years, were significantly associated with the presence of capsular invasion. The latter two variables were protective. In the multivariate analysis, only a tumor size over 5 mm was significantly associated with thyroid capsule involvement. Conclusions: A tumor size over 5 mm is associated with capsular invasion in PTMC.