Here, we describe a case of Sézary syndrome (SS) in a patient during treatment with baricitinib for seronegative RA. JAK inhibitors including baricitinib exert significant effects on various inflammatory diseases such as rheumatoid arthritis (RA) and atopic dermatitis. Https://doi.org/10.1093/ced/llac124 Baricitinib is an oral Janus kinase (JAK) inhibitor selective for JAK1 and 2, and inhibits the JAK/signal transducer and activator of transcription (STAT) signalling pathway. [Extracted from the article]