The combined effect of lactation and HIV infection results in increased energy demand for the lactating HIV infected women which culminate into increased rates of malnutrition. A suitable food supplement needs to be identified to meet this demand. Amaranthus cruentus L. flour is said to improve nutritional status markedly especially for people living with HIV. However, there is little information on how Amaranthus cruentus L. supplementation affects body composition and nutritional status of HIV infected mothers who are breastfeeding. The objective of this study was to assess the effect of Amaranthus cruentus L. supplementation on nutritional status and body composition of lactating HIV infected mothers attending Nyambene District Hospital. HIV positive women who opted to breastfeed for six months were recruited into the study. Eighty women were randomly assigned to the experimental group while eighty nine were randomly assigned to the control group. The experimental group received whole amaranth flour while the control group received maize/ wheat composite flour for a period of six months. For both groups, daily porridge rations were prepared using 300g of flour in three litres of water and were served in the morning, mid-morning and at four o’clock in the evening. Each serving provided 370 k/cal in the amaranth group and 362 k/cal in the maize composite group. Follow-up was done on the mothers to ascertain adherence to the feeding xvi instructions. Stable Isotope technique was used to assess body composition at baseline and at three and six months post intervention. Anthropometric measurements (Body Mass Index and Mid- Upper Arm Circumference) were also done at similar intervals. Based on BMI results, about half (47.3 %) of the women were undernourished at baseline. The results on nutritional status assessment revealed baseline equivalence on each of the indicators for every treatment arm. There was a significant positive relationship between fat free mass and Body Mass Index (Correlation coefficient=0.459, P<0.001), implying that fat free mass had significant contribution to Body Mass Index. Similarly there was a significant positive relationship between fat free mass and Mid –Upper Arm Circumference (Correlation coefficient=0.479, P<0.001). The relationship was much stronger compared to Body Mass Index and fat free mass. Mean Body Mass Index and Mid Upper Arm Circumference were significantly higher in the experimental group (P < 0.05) and a higher proportion of women increased in Fat Free Mass in the experimental group in comparison to the control group. Notably, all the variables had a higher mean in experimental group compared to control at both mid point and end point. Mean change in Body Mass Index and Midupper Arm Circumference was statistically significant (P<0.05) in experimental group. Mean change in Fat Free Mass was not statistically significant (P>0.05). xvii However, the change was higher in experimental group compared to control. In conclusion, the Amaranthus cruentus flour had a statistically significant positive effect on nutritional status of lactating HIV positive women compared to maize which had no statistically significant positive effect. In addition, the results revealed that Amaranthus cruentus is about three times more likely to improve the nutritional status of lactating HIV positive women than maize. This finding demonstrates that Amaranthus cruentus is more efficacious than maize in improving the nutritional status of lactating HIV positive women.