We report the case of an 11-month-old patient with a clinical diagnosis of infantile acute lymphoblastic leukemia and an MLL (11q23) rearrangement in 69% of nuclei, revealed with interphase fluorescence in situ hybridization (FISH). Routine chromosome analysis of the bone marrow showed a very subtle rearrangement involving the short arm of chromosome 10 and the long arm of chromosome 11 in the abnormal cells. To clarify the nature of this rearrangement, we hybridized the MLL break-apart probe to previously G-banded slides. The rearrangement was interpreted as a small inversion within the band 11q23, separating the 5' MLL from the 3' MLL region. This segment on the long arm of chromosome 11 containing the rearranged MLL locus was either inserted in or translocated to the short arm of chromosome 10 at approximately band 10p12. The inversion affecting MLL may have followed insertion or preceded it. Molecular characterization of this rearrangement was not possible, due to limited sample material. There have been previous reports of rearrangements of MLL with the MLLT10 (alias AF10) gene locus at 10p12, including an interstitial inverted insertion of 11q13q23 in one case and insertion of 11q14q23 at 10p12 in another. These both resulted in a large derivative chromosome 10 and transcription of an MLL/MLLT10 fusion product. To our knowledge, the novel and cryptic rearrangement detected in our patient has not been described previously. A follow-up study of the patient's bone marrow at the end of induction therapy showed no morphologic evidence of residual leukemia and both FISH and chromosome analyses were normal.