BACKGROUND: Change in low-density lipoprotein cholesterol (LDL-C) level after statin initiation varies widely among individuals, and in part may be because of factors shared by family members. METHODS AND RESULTS: We used the Danish national registers to identify 89 006 individuals who initiated statins between 2008 and 2018 and had LDL-C measured immediately before and after the start of treatment. Among these, we identified 5148 first-degree relatives and 3198 spouses. We decomposed the variation in attained LDL-C level after statin initiation by applying a mixed-effect model with 5 variance components (inter-family and inter-individual variance in pre-statin LDL-C level, inter-family and inter-individual variance in statin response, and residual variance). Results were presented as a percentage of the total variance explained by the different variance components. We found that half of the variation in attained LDL-C level after statin initiation consisted of variance in statin response, approximately one third of variance in pre-statin LDL-C level, and the remaining 10% to 15% of residual variance. While the inter-individual variance in statin response accounted for almost half of the LDL-C variation in both cohorts, the inter-family variance in statin response accounted for 3.3% among first-degree relatives and for 6.0% among spouses. CONCLUSIONS: Individual factors account for most of the variation in LDL-C level after statin initiation; factors affecting statin response common within spouses and first-degree relatives account for a similar share of variation. These results suggest a modest influence of shared genetics and shared familial environment on statin response.