The aim of this study was to determine if the distribution in vivo of CD4+CD45RA+/CD45RO− (naive), CD4+CD45RA+/CD45RO+ (Ddull) and CD4+CD45RO+ (memory) lymphocytes differs in malnourished infected and well-nourished infected children. The expression of CD45RA (naive) and CD45RO (memory) antigens on CD4+ lymphocytes was analysed by flow cytometry in a prospectively followed cohort of 15 malnourished infected, 12 well-nourished infected and 10 well-nourished uninfected children. Malnourished infected children showed higher fractions of Ddull cells (11·4 ± 0·7%) and lower fractions of memory cells (20·3 ± 1·7%) than the well-nourished infected group (8·8 ± 0·8 and 28·1 ± 1·8%, respectively). Well-nourished infected children showed increased percentages of memory cells, an expected response to infection. Impairment of the transition switch to the CD45 isoforms in malnourished children may explain these findings, and may be one of the mechanisms involved in immunodeficiency in these children.