Protein-energy malnutrition (PEM) is, together with infectious and parasitic diseases, a major cause of childhood illness in Africa. Diagnosis and treatment of PEM requires an accurate, simple and reliable method of assessing nutritional status from a blood sample. Plasma apolipoprotein (apo A1), prealbumin and albumin were measured in a group of Senegalese children suffering from PEM who had been hospitalized for refeeding, and in a group of control children. Statistical analysis of the results indicated that: (1) Plasma apo A1 was significantly correlated with prealbumin in assessing nutritional status (P less than 0.005 on day 8 of refeeding); (2) plasma apo A1 alone was sufficient for diagnosing and monitoring the dietary treatment of PEM; it was capable of detecting subclinical forms; (3) apo A1 could be used for differential diagnosis of forms of PEM; (4) plasma apo A1 concentration began to increase earlier (94% of control values at day 8) than did prealbumin (73% on day 8). We therefore propose apo A1 as an index of nutritional status in children living in areas where infectious and parasitic diseases are endemic.