In Mexico, adult-onset Still's disease (AOSD) is one of the causes of fever of unknown origin (FUO). The aim of this study is to describe a series of AOSD cases from a FUO cohort in order to know the clinical and biochemical characteristics of the cases, as well as to describe the neutrophil-lymphocyte index (NLI), which is a clinical marker of inflammation in autoimmune diseases.An observational study of 24 cases with AOSD; 72 % of them were women, the median age was 43 years (IQR 37.7-59.7), and the most frequent manifestations were classic rash (84 %) and arthralgia (100 %). All of them had tested negative for rheumatoid factor, antinuclear antibodies, and hyperferritinemia; 83 % had NLI > 3.08. The most used treatment was the combination of methotrexate with corticosteroids; seven patients required biological therapy, and one of them presented a hypersensitivity reaction.When there's FUO, the existence of AOSD should be suspected; also in the presence of rash, arthralgia, hyperferritinemia, and NLI > 3.08.