Definium Therapeutics (DFTX) had the highest return in March 2020 by a mid-cap US stock, returning 1,020%.

ASSET	MONTH	% RETURN
Definium Therapeutics (DFTX)	March 2020	1,020%
United Natural Foods (UNFI)	March 2020	40.15%
DHT (DHT)	March 2020	37.7%
Teekay Tankers (TNK)	March 2020	35.53%
Cal-Maine Foods (CALM)	March 2020	25.87%
Frontline (FRO)	March 2020	25.75%
CMBTECH (CMBT)	March 2020	21.03%
Vipshop (VIPS)	March 2020	19.85%
International Seaways (INSW)	March 2020	19.76%
AeroVironment (AVAV)	March 2020	17.62%
Syndax Pharmaceuticals (SNDX)	March 2020	16.7%
WD-40 (WDFC)	March 2020	16.35%
Tootsie Roll Industries (TR)	March 2020	15.4%
Sprouts Farmers Market LLC (SFM)	March 2020	15.11%
Enliven Therapeutics (ELVN)	March 2020	14.5%
Qiagen (QGEN)	March 2020	14.22%
PLDT Inc ADR (PHI)	March 2020	13.38%
Molina Healthcare (MOH)	March 2020	13.14%
Liquidia Technologies (LQDA)	March 2020	12.68%
Palomar (PLMR)	March 2020	12.39%
Arcutis Biotherapeutics (ARQT)	March 2020	11.82%
Repligen (RGEN)	March 2020	11.68%
Indivior PLC Ordinary Shares (INDV)	March 2020	9.69%
RLI (RLI)	March 2020	9.39%
ConAgra Foods (CAG)	March 2020	9.23%

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