Background: Rearranged during transfection (RET) gene alterations are the oncogenic driver in diverse tumor types, including RET fusions identified in 1-2% of non-small cell lung cancers (NSCLC). While RET-selective tyrosine kinase inhibitors (TKIs) selpercatinib and pralsetinib are effective, acquired drug resistance remains a challenge. Here, we report the initial results from the RETgistry, an international consortium aimed at elucidating mechanisms of resistance to RET TKIs across RET-altered solid tumors. Methods: This was a retrospective analysis performed across 16 institutions. Patients (pts) were eligible if they had advanced solid tumor harboring an oncogenic RET alteration, received ≥1 RET TKI with disease progression, and underwent resistant tumor or liquid biopsy for next-generation sequencing (NGS). Results: A total of 103 time-distinct biopsies (62 tissue, 30 plasma, 11 paired tissue/plasma) were included in analysis, obtained from 88 pts with progression on a RET-selective TKI [selpercatinib (n = 70), pralsetinib (n = 14), selpercatinib and pralsetinib sequentially (n = 4)]. Pts had the following tumor types: 72 NSCLC (69% KIF5B-RET, 21% CCDC6-RET, 10% other RET fusion), 13 medullary thyroid cancer (TC) (54% RET M918T, 46% other RET mutation), and 2 papillary and 1 anaplastic TC (all, RET fusion). Median age was 58 (range, 21-86); 48% were male with 89% never/light smokers. Median duration of RET TKI preceding biopsies [first-line, n = 32 (31%); second-line, n = 42 (41%); third-/greater-line, n = 29 (28%)] was 16.5 months (mos) (95% CI, 14.0-19.6); median PFS was 14.1 mos (95% CI, 9.3-17.0). Resistant biopsies were obtained at median 15.0 mos from TKI initiation (range, 1.8-58.8). Acquired RET mutations were detected in 14 (14%), most common being G810 substitutions in 12 (12%). Potential off-target resistance gene alterations identified in 43 cases (42%) included MET amplification (14%), BRAF V600E or fusion (2%), KRAS gain or mutation (5%), ERBB2 amplification (2%), EGFR amplification (3%), ROS1 fusion (1%), and activating PIK3CA mutation or PTEN loss (4%). No resistant lung cancer biopsy demonstrated small cell transformation. The duration of TKI therapy (HR 0.64, p = 0.11) or PFS (HR 0.75, p = 0.33) did not differ according to the presence of on-target vs off-target resistance. NGS analyses of pre-RET-selective TKI tumors (n = 92) revealed frequent co-occurring alterations in TP53 (29%) and CDKN2A/B (12%). Conclusions: On-target resistance to RET inhibition due to acquired RET mutations was less common than off-target resistance, identified in 14%. The majority of RET TKI resistance is mediated by off-target mechanisms such as bypass receptor tyrosine kinase activation. Further studies are warranted to enable the development of therapeutic strategies to address resistance in pts with RET-altered tumors. The RETgistry accrual and data analyses continue.