As a part of the routine medical care, hematologic data (ie, complete blood cell count) were collected from 21 consecutive patients with LM not undergoing active treatment for more than 7 days, from January 1, 1999, through December 31, 2004, at Children’s Hospital and Regional Medical Center, Seattle, Wash. In patients with a history of unexplained recurrent LM inflammation or swelling, a lymphocyte differential count was obtained. Inclusion criteria were an established or new diagnosis of LM based on pathological tissue findings or results of a radiographic evaluation between 1994 and 2004. Pertinent clinical data recorded for analysis included patient age at the time of testing, history of LM inflammation, treatment type (ie, medical, surgical, or sclerotherapy), LM stage, radiographic LM description (ie, macrocystic, microcystic, or mixed), and, when available, histologic description. The LM stage was determined according to the criteria of de Serres et al.10 Stage 1 has unilateral infrahyoid disease; stage 2, unilateral suprahyoid disease; stage 3, unilateral infrahyoid and suprahyoid disease; stage 4, bilateral suprahyoid disease; and stage 5, bilateral infrahyoid and suprahyoid disease. A detailed lymphocyte differential count that included the absolute number and percentage of T cells (T4 [CD3+ and CD4+]) and T8 cells [CD3+ and CD8+]), B cells (CD19+), and natural killer (NK) cells (CD16+ and CD3−) was obtained (Epics XL flow cytometer; Coulter Beckman, Fullerton, Calif). Cell surface markers were used to specifically identify T4, T8, B, and NK cells. All normal values for lymphocyte counts and hematopoietic tests were adjusted for age. To be considered lymphocytopenic, a patient’s lymphocyte counts had to be less than the lower limit of the reference range for their age (ie, T4 cell absolute number for ages 5-10 years, 300 cells/μL). For purposes of analysis, lymphocytopenia was defined as an absolute lymphocyte count of less than 1500 cells/μL. In the detailed lymphocyte differential count, lymphocytopenia was defined as follows: T4 cells less than 300 cells/μL, T8 cells less than 300 cells/μL, B cells less than 200 cells/μL, and NK cells less than 150 cells/μL.