Many cases deal with the use of Federal tax returns in non-tax cases. The inherent danger in subjecting tax returns to discovery at pre-trial and admission at trial, is the unnecessary disclosure of confidential information. Yet, when income tax returns are material and relevant to the claims of the parties litigant, a contention of privileged communication should not be used to defeat justice.
Laurence Glazer, Federal Tax Returns as Evidence in Non-Tax Cases, 14 Clev.-Marshall L. Rev. 164 (1965)