Peripheral T- and NK-cell lymphomas (PT/NKCLs) are relatively rare, and few studies have validated the International Prognostic Index (IPI) for PT/NKCLs in prospective clinical trials. Histopathological specimens from 136 patients, enrolled in six prospective multicenter trials of doxorubicin-containing regimens, with PT/NKCLs were reviewed by six hematopathologists following the WHO classification. This combined analysis demonstrated that the IPI was not predictive of prognosis for patients with PT/NKCLs as previously shown by GELA. In a univariate analysis, low total serum protein (TP) and albumin levels, gastrointestinal tract involvement, and histologic subtype (extranodal NK/T-cell lymphoma, nasal type, and peripheral T-cell lymphoma, unspecified) were significantly associated with reduced survival. In a multivariate analysis, TP (p = 0.004) and histologic subtype (p = 0.024) remained significant. We discuss the need to establish the importance and meaning of TP and to develop new strategies for patients with PT/NKCLs allowing for TP, especially with worse histologic subtypes.