Recombination of antigen receptor gene elements (V, D and J) that code for Ig and TCR variable regions is defective in the scid mouse mutant. However, the defect is not absolute. Developing scid lymphocytes occasionally succeed in forming a productive VDJ and VJ rearrangement at two critical loci (e.g., IgH/IgL or TCR beta/TCR alpha), and given the appropriate stimuli, differentiate into clones of functional B or T cells. Scid mice with detectable B and/or T lymphocytes are referred to as leaky. This review considers the nature and possible basis of B and T cell leakiness in scid mice.