Himax Technologies (HIMX) had the highest return in December 2021 by a mid-cap US stock, returning 58.2%.

ASSET	MONTH	% RETURN
Himax Technologies (HIMX)	December 2021	58.16%
Hagerty (HGTY)	December 2021	38.88%
Aehr Test Systems (AEHR)	December 2021	38.65%
Silicon Motion Technology (SIMO)	December 2021	36.19%
Applied Digital (APLD)	December 2021	32.18%
TG Therapeutics (TGTX)	December 2021	30.58%
Jackson Financial (JXN)	December 2021	30.27%
Laureate Education (LAUR)	December 2021	26.53%
Regencell Bioscience (RGC)	December 2021	26.14%
Karooooo (KARO)	December 2021	24.98%
Pampa Energia SA ADR (PAM)	December 2021	24.95%
Trump Media & Technology (DJT)	December 2021	24.05%
LG Display (LPL)	December 2021	23.46%
Arcutis Biotherapeutics (ARQT)	December 2021	23.45%
ACADIA Pharmaceuticals (ACAD)	December 2021	22.78%
Alliance Resource Partners (ARLP)	December 2021	22.48%
Penguin Solutions (PENG)	December 2021	22%
Aurinia Pharmaceuticals (AUPH)	December 2021	21.65%
Howard Hughes (HHH)	December 2021	21.18%
Cushman & Wakefield (CWK)	December 2021	20.61%
Inter Parfums (IPAR)	December 2021	20.43%
Century Aluminum (CENX)	December 2021	20.17%
Powell Industries (POWL)	December 2021	19.88%
Builders FirstSource (BLDR)	December 2021	19.79%
Nektar Therapeutics (NKTR)	December 2021	19.66%

Mid-cap stocks are treated as having a market cap between $2B and $10B.