PURPOSE: Weight gain is a known adverse event (AE) of alectinib. This study evaluates the progression of actual weight gain over time and explores its association with baseline characteristics. METHODS: A pooled analysis of individual patient data from four clinical trials (ALEX, J-ALEX, ALUR, and ML29453) was conducted. Actual weight gain was calculated as the percent change from baseline. A linear mixed model estimated weight change over time and associations between clinical characteristics and weight change. RESULTS: Follow-up weights were available for three trials (J-ALEX, ALUR, and ML29453) and missing for ALEX. In total, 2,622 weights were recorded in the first year (N = 302). At baseline, 13.6% of the Japanese population were underweight and 5.0% in the Western population. Actual weight gain of any grade was substantially higher than reported AE rates (49% CONCLUSION: Weight gain is under-reported as AE in trials. Actual weights showed ≥10% weight gain in 18% of patients. Clinicians should be aware of this AE, emphasizing the importance of timely identification and monitoring weight. Identifying predictors for weight gain remains challenging.