To identify differences in groups of children with special healthcare needs (CSHCN) identified as underinsured by two alternate definitions and discuss implications for policy decisions based on using one definition over another. Secondary data from the National Survey of CSHCN 2005/2006 were analyzed. Only CSHCN who were continuously-insured for 12 months were included in analyses. We identified groups of underinsured CSHCN using two general definitions ("economic" and "attitudinal") and three mutually-exclusive groups (identified by both definitions, identified by attitudinal but not economic, and identified by economic but not attitudinal). Key variables included demographics and condition characteristics. Different underinsurance rates were identified [attitudinal = 30.9 % (n = 11,470); economic = 22.7 % (n = 8,447)] with fair agreement by kappa score (κ = 0.3194; Z = 65.91; p > 0.0001). Differences across mutually-exclusive groups included family income ≥400 % FPL (attitudinal only = 34.2 %, economic only = 16.3 %, both = 18.4 %; p < 0.001) and high severity (attitudinal only = 42.5 %, economic only = 68.5 %, both = 69.9 %; p < 0.001). CSHCN who needed equipment/supplies/home health (OR = 1.31, p < 0.001) had increased odds of being identified as underinsured by the economic, but not attitudinal definition. CSHCN with private insurance had increased odds of being identified by attitudinal only or both definitions, but not by economic only (AO: OR = 1.41, p < 0.001; BOTH: OR = 2.36, p < 0.001). Despite overlap between the two definitions, choosing either one excludes some CSHCN, potentially underestimating the extent of underinsurance and masking important findings related to specific conditions characteristics. A definition that comprehensively identifies and describes underinsurance is vital to translating health insurance coverage expansion into benefit packages that meet complex health and service needs. © 2012 Springer Science+Business Media New York.