A study was designed to assess the efficacy of a recombinant feline omega interferon (rFeIFNw) in the treatment of symptomatic FeLV or FeLV and FIV positive cats. In a multicentric, controlled, randomised and double blind clinical field trial, 48 FeLV or FeLV and FIV tested cats were included with general clinical signs and/or chronic granulomatous stomatitis. 28 cats were subcutaneously administered 1 million units (MU) interferon per kg b.w. once a day for 5 days and 20 cats received a placebo. Symptomatic treatment including antibiotherapy was allowed in all cats. Cats were clinically observed for 6 months and death date was recorded if applicable. Efficacy was assessed through the survival probabilities which were calculated by the KAPLAN-MEIER method and compared by the LOGRANK test. 13 (46%) cats died during the observation period in rFeIFNw group and 15 (75%) in the placebo group. Comparison of the survival curves showed a statistically significant difference between groups (p = 0.0331). Safety of the product based on clinical examination was excellent. After 5 SC injections of 1 MU/kg rFeIFNw, the survival probability over a 6-month period of FeLV or FeLV and FIV symptomatic cats was 2.2 times higher in the treated cats.