Acm Research (ACMR) had the highest return in March 2018 by a mid-cap US stock or mid-cap US stock, returning 83.3%.

ASSET	MONTH	% RETURN
Acm Research (ACMR)	March 2018	83.31%
CleanSpark (CLSK)	March 2018	74.45%
Ameresco (AMRC)	March 2018	60.49%
RadNet (RDNT)	March 2018	44%
Addus HomeCare (ADUS)	March 2018	42.04%
Verona Pharma PLC ADR (VRNA)	March 2018	41.14%
Enphase Energy (ENPH)	March 2018	37.65%
Burford Capital (BUR)	March 2018	36.58%
Sunrun (RUN)	March 2018	33.88%
Xenon Pharmaceuticals (XENE)	March 2018	30.67%
Crocs (CROX)	March 2018	27.45%
Ocular Therapeutix (OCUL)	March 2018	24.24%
Walker & Dunlop (WD)	March 2018	23.1%
Korn Ferry (KFY)	March 2018	23.08%
Apellis Pharmaceuticals (APLS)	March 2018	22.82%
American Superconductor (AMSC)	March 2018	22.78%
ADMA Biologics (ADMA)	March 2018	22.02%
Floor & Decor (FND)	March 2018	20.51%
Delek US Energy (DK)	March 2018	19.79%
Abercrombie & Fitch (ANF)	March 2018	18.86%
CarGurus (CARG)	March 2018	18.73%
PagSeguro Digital (PAGS)	March 2018	18.16%
Supernus Pharmaceuticals (SUPN)	March 2018	17.89%
First Majestic Silver (AG)	March 2018	17.5%
AXIS Capital (AXS)	March 2018	17.4%

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