Acm Research (ACMR) had the highest return in January 2020 by a mid-cap US stock, returning 85.5%.

ASSET	MONTH	% RETURN
Acm Research (ACMR)	January 2020	85.48%
Immunitybio (IBRX)	January 2020	65.01%
Applied Digital (APLD)	January 2020	51.16%
Skeena Resources (SKE)	January 2020	39.82%
Almonty Industries Inc Common Shares (ALM)	January 2020	39.75%
Apellis Pharmaceuticals (APLS)	January 2020	32.78%
Kiniksa Pharmaceuticals (KNSA)	January 2020	32.4%
Liquidia Technologies (LQDA)	January 2020	32.35%
Denali Therapeutics (DNLI)	January 2020	31.89%
Bill Com (BILL)	January 2020	31.18%
TG Therapeutics (TGTX)	January 2020	27.9%
Bath & Body Works (BBWI)	January 2020	26.9%
iRhythm Technologies (IRTC)	January 2020	24.68%
VNET Group Inc DRC (VNET)	January 2020	23.36%
Impinj (PI)	January 2020	22.64%
Zai Lab (ZLAB)	January 2020	22.13%
Sunrun (RUN)	January 2020	21.73%
Ultragenyx (RARE)	January 2020	21.7%
SLM (SLM)	January 2020	21.6%
Soleno Therapeutics (SLNO)	January 2020	21.4%
Ringcentral (RNG)	January 2020	20.93%
DRDGOLD Limited ADR (DRD)	January 2020	20.68%
Plug Power (PLUG)	January 2020	20.4%
Signet Jewelers (SIG)	January 2020	19.63%
Enphase Energy (ENPH)	January 2020	19.53%

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