Thirty-six children (age range, 18-30 months) born to HIV-1-infected mothers were studied for speech development by matching 18 infected with 18 non-infected subjects for age, sex and socioeconomic status. All the children were in good health. Each child was given three comprehension and three production tasks. In addition, each child's mean length of utterance (MLU) was obtained by observation of natural child-parent interactions. The development quotients (DQ) were assessed by Brunet-Lézine's tests. Infection significantly affected children's MLU, the infected children being less advanced than those non-infected. Both infected and non-infected children progressed in language acquisition from the second to the third year of age, but infected children had significantly greater production difficulty than non-infected children in the second year of life. The matched subjects design adopted gives some strength to the conclusion that HIV-1 infection impairs the genesis rather than the later development of language in infected but not ill children.