We observed two sisters with ataxia telangiectasia, one of whom developed an atypical subacute lymphocytic leukemia characterized by atypical lymphocytes and absence of palpable lymphadenopathy or hepatosplenomegaly. The lack of organomegaly in this patient may have been due to the underlying ataxia telangiectasia, which was associated with lymphoid hypoplasia. Cytogenetic studies showed a marker chromosome 14 [t(14q11:14q34)] in both patients. The sister with leukemia had other complex chromosomal aberrations in addition to the marker chromosome 14 that were stable for more than 14 mo before the patient's death from complicating infection. The development of atypical T cell leukemia has not been previously described in ataxia telangiectasia. This case further illustrates the interesting interrelationships amoung immunosuppressed states, development of lymphoid malignancy, and an emerging pattern of a propensity to chromosome 14 abnormalities in various lymphoid malignancies.