The objective was to investigate the effects of omega-3 fatty acids, known for their anti-inflammatory effects, on time to lymphoma progression and survival in the TLL mouse, a strain genetically prone to developing aggressive T-cell lymphoma. Compared to mice fed a standard diet, TLL mice fed omega-3 (menhaden fish oil) experienced a significant delay in disease progression and were more likely to remain alive and symptom free during the first 8 months of the study. In contrast, omega-6 supplementation (corn oil) did not significantly affect lymphoma progression. Irrespective of diet, all mice eventually progressed, and 1-year survival was not different between the groups. Immunological analysis demonstrated a significantly altered B-cell compartment and fewer NK cells in healthy C57Black6 mice fed omega-3, compared to controls. In conclusion, a diet rich in omega-3 fatty acids delays lymphoma development in the TLL mouse possibly by mechanisms that include complex effects on immune function.