Information about health care use and costs of cutaneous T-cell lymphoma (CTCL) patients is limited, particularly in a European setting. Methods: In this population-wide study we set out to investigate prevalence, and trends in health care use in two CTCL subtypes, mycosis fungoides (MF) and Sézary syndrome (SS) over a time period of 19 years in 1998-2016 by using a nation-wide patient register containing data on all diagnosed MF and SS cases in Finland. Results: The prevalence of diagnosed MF

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... nd SS rose from 2.04 to 5.38/100000, and from 0.16 to 0.36/100000 for MF and SS respectively during 1998-2016. We found a substantial decrease in inpatient treatment of MF/SS in the past two decades with a mean of 2 inpatient days/patient/year due to MF/SS in 2016, while the mean numbers of MF/SS related outpatient visits remained stable at 8 visits/year/patient. Most MF/SS-related outpatient visits occurred in the medical specialty of dermatology. In a ten-year follow-up after MF/SS diagnosis, the main causes for outpatient visits and inpatient stays were MF/SS itself, other cancers, and other skin conditions. Also cardiovascular disease and infections contributed to the number of inpatient days. Mean total hospital costs decreased from 11, 600 eur/patient/year to 3600 eur/patient/year by year 4 of the follow-up, and remained at that level for the remainder of the 10-year follow-up. MF/SS accounted for approximately half of the hospital costs of these patients throughout the follow-up. Conclusions: The nearly 3-fold increase in prevalence of diagnosed MF/SS during 1998-2016 puts pressure on the health care system, as this is a high-cost patient group with a heavy burden of comorbidities. The challenge can be in part answered by shifting the treatment of MF/SS to a more outpatient-based practice, and by adapting new pharmacotherapy, as has been done in Finland.