Background: RW data on PD pattern and safety of 1L osimertinib in EGFR mut aNSCLC are currently lacking. Methods: This is an observational, multicenter study enrolling EGFR mut aNSCLC receiving 1L osimertinib between November 2018 and 2020. Primary endpoints were treatment outcomes: median progression free survival (mPFS), time to treatment failure (mTTF) and PD patterns. Safety profile was also recorded as a secondary endpoint. PD was classified as isolated (single lesion PD), oligoprogression (oligoPD; PD in ≤ 3 lesions in ≤ 2 sites), systemic all others. Results: At data cut-off (January 5, 2021), 82 pts receiving osimertinib were included in 3 centres: 52 (63.4%) were female, 46 (56.1%) never smokers. Baseline EGFR mutations were exon 19 deletion in 39 (47.6%) pts, L858R in 36 (43.9%) pts, others in 7 (8.6%) pts. ECOG performance status was 0-1 in 65 (79.3%) pts. Number (N) of metastatic sites at diagnosis was <3 in 52 (63.4%) pts, ≥ 3 in 30 (35.6%) pts; 27 (32.9%) pts had brain metastases. Median follow-up was 11.2 months (mo). Response rate was 68.3%, disease control rate 86.6%. mPFS was 22.0 mo (95%CI, 11.4-32.5), mTTF 25.3 mo (95%CI, 18.9-not calculable). PD was reported in 28 (34%) pts; median N of PD sites was 2 (range 1-7); the most frequent PD sites were lung (N = 19, 67.9%), bone (N = 11, 39.3%) and brain (N = 7, 25%). New PD sites were present in 11 (39.3%) pts; median N = 1 (range 1-4). Isolated PD was observed in 5 (17.9%) pts, oligoPD in 6 (24.4%) and systemic in 16 (57.1%). 9 cases underwent tissue and/or liquid rebiopsy at PD. Druggable resistance mechanisms were identified in 5 pts: MET amplification (amp) (N = 3), MET amp/EGFR amp (N = 1) and HER2 amp (N = 1). The most frequent any grade (G) adverse events (AEs) were diarrhea (N = 33, 40.2%), rash (N = 32, 39.0%), paronychia (N = 22, 26.8%) and creatinine increase (N = 25, 30.5%). The most frequent G3/4 AEs were thromboembolic events (N = 6, 7.3%), diarrhea (N = 4, 4.9%), arterial thromboembolism (N = 2, 2.4%) and rash (N = 2, 2.4%). Conclusions: Osimertinib confirmed efficacy and safety in RW. Thromboembolic events were observed more frequently than previously reported. The study is still ongoing in order to recruit a larger patient population.