Objectives: To assess (a) the impact of daily preventive zinc tablets (7 mg; PZ), zinc-containing multiple micronutrient powder (10 mg zinc, and 13 other micronutrients; MNP) or placebo, delivered for 9 months, on Insulin-like Growth Factor 1 (IGF1) and IGF Binding Protein 3 (IGFBP3) among Laotian children 6–23 months, and (b) whether the effects of PZ and MNP on length-for-age z-scores (LAZ) and weight-for-age z-scores (WAZ) are modified by baseline IGF1 and IGFBP3. Design: A double-blind, placebo-controlled trial (N = 419). Methods: Plasma IGF1 and IGFBP3 concentrations at baseline and 36 weeks were analyzed by automated chemiluminescent assay. Anthropometry was assessed at baseline, at 18 and 36 weeks. Intervention effects were estimated using ANCOVA. Results: At 36 weeks, geometric mean IGF1 (~39.0–39.2 ng/mL; p = 0.99) and IGFBP3 (2038–2076 ng/mL; p = 0.83) did not differ by group. At 18 weeks (but not at 36 weeks), LAZ in the PZ group (−1.45) was higher than the MNP (−1.70) and control (−1.55) groups (p = 0.01) among children in the highest baseline IGF1 tertile (p for interaction = 0.006). At 36 weeks (but not at 18 weeks), WAZ in the PZ group (−1.55) was significantly higher than the MNP (−1.75) and control (−1.65) groups (p = 0.03), among children in the lowest baseline IGFBP3 tertile (p for interactions = 0.06). Conclusions: Although IGF1 and IGFBP3 did not respond to PZ and MNP, baseline IGF1 and IGFBP3 significantly modified the impact of PZ on linear and ponderal growth, suggesting that IGF1 bioavailability may drive catch-up growth in zinc-supplemented children