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Background: Anti‐myelin‐associated glycoprotein (MAG) neuropathy is a debilitating demyelinating polyneuropathy with no approved therapies. Our primary objective was to ascertain lenalidomide safety and maximum tolerated dose (MTD) in anti‐MAG neuropathy. Methods: This phase 1b, open‐label, single‐arm, dose‐finding trial was conducted from 2019 through 2022. The original design included a dose‐escalation/extension phase followed by a dose‐expansion phase. Three doses of lenalidomide were evaluated: 10, 15, and 25 mg. The main outcome was the MTD. Results: Eleven patients enrolled (10 men), with a mean age of 67.6 years (SD = 6.18, range 58–77 years) and mean disease duration of 8.5 years (SD = 10.9, range 1–40 years). The study terminated early due to higher‐than‐expected non‐dose‐limiting toxicity venous thromboembolism (VTE) events. The calculated MTD was 25 mg (posterior mean of toxicity probability was 0.01 with a 95% credible interval of 0.00, 0.06), but a recommended phase 2 dose of 15 mg was advised. For secondary exploratory outcomes, only EQ‐5D (−0.95, 95% CI −1.81 to −0.09) and total IgM (−162 mg/dL, 95% CI −298 to −26) showed signs of improvement by month 12. Conclusions: Lenalidomide was associated with higher‐than‐expected VTE events in anti‐MAG neuropathy patients, despite a calculated MTD of 25 mg. A recommended phase 2 dose of 15 mg was advised. Lenalidomide did not improve disability or impairment at 12 months, although this study was not powered for efficacy. The risks of long term lenalidomide may outweigh benefit for patients with anti‐MAG neuropathy. Any future efficacy study should address VTE risk, as current myeloma guidelines appear inadequate. Trial Registration: Lenalidomide in Anti‐MAG Neuropathy: Phase 1b Study, ClinicalTrials.gov Identifier: NCT03701711, https://clinicaltrials.gov/ct2/show/NCT03701711. First submitted October 10, 2018. First patient enrolled in January 2019. [ABSTRACT FROM AUTHOR]