Plug Power (PLUG) had the highest return in April 2017 by a mid-cap US stock or mid-cap US stock, returning 63.5%.

ASSET	MONTH	% RETURN
Plug Power (PLUG)	April 2017	63.5%
CleanSpark (CLSK)	April 2017	49.5%
Bancorp (TBBK)	April 2017	26.97%
Sunoco (SUN)	April 2017	26.1%
Cytokinetics (CYTK)	April 2017	25.67%
R1 RCM (RCM)	April 2017	25.48%
PTC Therapeutics (PTCT)	April 2017	23.85%
Impinj (PI)	April 2017	23.21%
LeMaitre Vascular (LMAT)	April 2017	20.74%
Globalstar (GSAT)	April 2017	19.38%
Ionis Pharmaceuticals (IONS)	April 2017	18.61%
PAR Technology (PAR)	April 2017	18.42%
Melco Resorts & Entertainment (MLCO)	April 2017	16.69%
Merit Medical Systems (MMSI)	April 2017	16.61%
Blueprint Medicines (BPMC)	April 2017	16.25%
Ascendis Pharma A-S (ASND)	April 2017	15.86%
CBIZ (CBZ)	April 2017	15.81%
Hilton Grand Vacations (HGV)	April 2017	15.61%
Grifols SA ADR (GRFS)	April 2017	15.05%
Brinks (BCO)	April 2017	14.87%
Avient (AVNT)	April 2017	14.68%
Match (MTCH)	April 2017	14.29%
Boise Cascad Llc (BCC)	April 2017	14.23%
Rush Enterprises A (RUSHA)	April 2017	13.88%
Ollie's Bargain Outlet Hldg (OLLI)	April 2017	13.82%

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