Background: Recurrent infections and inflammation contribute to growth faltering in low-income countries. n-3 (omega-3) Long-chain polyunsaturated fatty-acids (LC-PUFAs) may improve immune maturation, resistance to infections, and growth in young children who are at risk. Objective: We evaluated the independent and combined effects of fish oil (500 mg n-3 LC-PUFAs/d) supplementation to lactating mothers and their breastfed children, aged 6-24 mo, on child morbidity, systemic inflammation, and growth in southwest Ethiopia. Design: A 4-arm double-blind randomized controlled trial was conducted by enrolling 360 mother-infant pairs with infants 6-12 mo old. Study arms were both the lactating mother and child receiving fish oil intervention (MCI), only the lactating mother receiving fish oil intervention and child receiving placebo control (MI), only the child receiving intervention and mother receiving placebo control (CI), and both mother and child receiving a placebo supplement or control (C). The primary study outcome was linear growth using monthly changes in length-for-age z score. Anthropometric measurements were taken monthly, and hemoglobin, C-reactive protein, and blood LC-PUFAs were measured at baseline and after 6 and 12 mo of follow-up. Weekly morbidity surveillance was conducted throughout the study. Results: Fish-oil supplementation significantly increased blood n-3 LC-PUFA concentration (P < 0.01) and decreased the arachidonic acid:(docosahexaenoic acid + eicosapentaenoic acid) ratio (P < 0.001) in all intervention arms. No significant intervention effect was found on linear growth, morbidity, or systemic inflammation. Compared to the control group, a small positive effect on monthly changes in weight-for-length z scores was found in the CI arm (effect size: 0.022/mo; 95% CI: 0.005, 0.039/mo; P = 0.012) and the MCI arm (effect size: 0.018/mo; 95% CI: 0.001, 0.034/mo; P = 0.041). Conclusions: n-3 LC-PUFA supplementation of lactating mothers and children did not affect child linear growth and morbidity in a low-income setting. n-3 LC-PUFA supplementation given directly to children modestly increased relative weight gain.