Hut 8 Corp Common Stock (HUT) had the highest return in April 2019 by a mid-cap US stock, returning 73.5%.

ASSET	MONTH	% RETURN
Hut 8 Corp Common Stock (HUT)	April 2019	73.53%
Impinj (PI)	April 2019	70.17%
Alphatec (ATEC)	April 2019	57.99%
Riot Blockchain (RIOT)	April 2019	56.38%
Sonic Automotive (SAH)	April 2019	34.87%
Power Solutions International, Inc Common Stock (PSIX)	April 2019	32.87%
Silicon Laboratories (SLAB)	April 2019	31.57%
BRF SA ADR (BRFS)	April 2019	31.38%
Star Bulk Carriers (SBLK)	April 2019	30.29%
Scorpio Tankers (STNG)	April 2019	28.9%
Garrett Motion (GTX)	April 2019	27.03%
Bancorp (TBBK)	April 2019	25.74%
360 DigiTech (QFIN)	April 2019	25.44%
Globalstar, Inc Common Stock (GSAT)	April 2019	24.3%
Axsome Therapeutics (AXSM)	April 2019	23.99%
Frontline (FRO)	April 2019	23.34%
Customers Bancorp (CUBI)	April 2019	22.9%
Ufp Industries (UFPI)	April 2019	22.51%
Herc (HRI)	April 2019	22.23%
Appfolio (APPF)	April 2019	21.07%
Lithia Motors (LAD)	April 2019	21.04%
Pilgrims Pride (PPC)	April 2019	20.62%
Oceaneering International (OII)	April 2019	20.53%
WillScot Mobile Mini (WSC)	April 2019	20.48%
ELF Beauty (ELF)	April 2019	20.21%

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