OBJECTIVES: To develop and validate a risk calculator for the prediction of anemia occurring after 8 weeks of TT using BOC+PR among Hepatitis C Virus (HCV) genotype 1 patients based on risk factors. METHODS: Data for all randomized HCV genotype 1 patients starting therapy with BOC+PR and who received at least one dose of BOC in three phase 3 clinical trials (PN05101, PN5216 and PN5514) were included. The outcome of interest was the prediction of anemia, defined as hemoglobin <10 g/L after 12 weeks of TT with BOC+PR (this included 4 weeks of lead-in treatment using PR). Logistic regression was used to develop the risk calculator model by analyzing the association of each variable with the likelihood of developing anemia while on treatment. Baseline variables were included as covariates in the model. Linearity assumptions were relaxed with the use of restricted cubic splines. Bootstrapping, with 1000 resamples, were used in conjunction with estimation of discrimination and calibration. RESULTS: Following a stepdown procedure that eliminated predictors that did not contribute to the overall model concordance index, nine variables remained in the final model: age, hemoglobin, gender, cirrhosis, hematologic counts, Alkaline phosphatases levels, and creatinine.  This model had a boostrap corrected concordance index of 0.775. The model was cross-validated by sequentially omitting each of the three randomized trials from the model development and using the omitted trial as a test set; the concordance indices following this procedure ranged from 0.75 to 0.85. Calibration of the model, assessed graphically, indicated reasonably close agreement between predicted and observed proportions.  Calibration held following trial cross validation as well. CONCLUSIONS: The model calibrated well and demonstrated good predictive ability. This tool may be useful for identifying and subsequently managing HCV patients at relatively high risk for developing anemia if treated with BOC+PR.