To determine if the outcomes of patients with ILC co-occurring with LCIS are similar to pure ILC and if the presence of LCIS is a prognostic factor for ILC. In an observational, population-based investigation using data from the MD Anderson breast cancer prospectively collected electronic database, we analyzed patients with a diagnosis of stage I-III ILC. Patients were divided into 2 groups: those with ILC with co-occurring ipsilateral LCIS (ILC+LCIS) and those with pure ILC without a histologically detected co-occurring ipsilateral LCIS (ILC alone). We obtained data on demographics, pathologic tumor size (pT), pathologic lymph node (pN) involvement, estrogen (ER), progesterone (PR) receptor status, HER2 status, Ki67, treatment received, distant recurrence-free and overall survival (DRFS, OS). We identified 4,217 patients with stage I-III ILC treated at MD Anderson between 1966 and 2021. 45% of cases (n = 1,881) had co-existing LCIS. Statistically and numerically, ILC alone tended to associate with pT4 and pN3 stage (P < 0.001), ER/PR negativity (P = 0.0002), HER2 positivity (P = 0.010), higher Ki67 (P = 0.005), non-classical ILC subtype (P = 0.04) and more exposure to neoadjuvant chemotherapy (P = 0.0002) compared to the ILC+LCIS group. The median follow-up time was 6.5 years. Patients with ILC+LCIS had better median DRFS (16.8 vs 10.1 years, Hazard ratio (HR) 0.55, 95% confidence interval (CI) 0.50 – 0.60, P <0.0001) and better median OS (18.9 vs 13.7 years, HR 0.62, 95% CI 0.56 – 0.69; P <0.0001). Multivariate analysis showed the absence of LCIS to be an independent poor prognostic factor along with a higher pT stage and higher pN stage for DRFS and OS. The findings of this study suggests that the absence of ipsilateral LCIS with ILC is an independent poor prognostic factor and that further studies are warranted to understand this phenomenon. The data that support the findings of this study are available from the corresponding author, upon reasonable request. •LCIS co-occurs with ipsilateral ILC in 45% of the cases at the time of diagnosis.•Pure ILC tend to be associated with higher stages at diagnosis compared to ILC+LCIS.•Absence of LCIS at the time of diagnosis of ILC is a poor prognostic factor.