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To determine predictors of substantial weight gain (SWG) during treatment of bipolar disorder with olanzapine, data were pooled from 4 long-term randomized, multicenter studies in patients with bipolar mania or mixed mania (N = 948 at initiation of olanzapine). SWG was defined as gaining 5 kg or 7% of initial weight in 30 ± 2 weeks. Logistic regression estimated odds ratios associated with early weight gain and baseline risk factors for predicting SWG. A classification system to identify patients at risk for SWG was constructed by recursive data partitioning. Baseline characteristics significantly associated with SWG included younger age, nonwhite ethnicity, lower body mass index (BMI), nonrapid cycling, and psychotic features. Weight gain of 2 or more kg in the first 3 weeks of therapy predicted SWG by 30 weeks (sensitivity = 57%; specificity = 71%). A classification system with thresholds for early weight gain, baseline BMI, and ethnicity further improved SWG predictability (sensitivity = 79%; specificity = 70%). In conclusion, patients with bipolar disorder who gained 2 to 3 kg during the first 3 weeks of treatment with olanzapine, SWG was predicted after 30 weeks of treatment. Patients with less pronounced early weight gain might still be at risk for later SWG if they have close to normal BMI (≤27 kg/m2) at treatment initiation.