Iovance Biotherapeutics (IOVA) had the highest return in May 2019 by a mid-cap US stock, returning 42.9%.

ASSET	MONTH	% RETURN
Iovance Biotherapeutics (IOVA)	May 2019	42.92%
Kratos Defense & Security Solutions (KTOS)	May 2019	38.94%
TransMedics (TMDX)	May 2019	34.27%
Kodiak Sciences (KOD)	May 2019	34.06%
Stagwell (STGW)	May 2019	33.33%
Arrowhead Pharmaceuticals (ARWR)	May 2019	31.72%
Axsome Therapeutics (AXSM)	May 2019	29.35%
Banco Macro SA B ADR (BMA)	May 2019	29.11%
Transportadora de Gas del Sur SA ADR (TGS)	May 2019	28.71%
Zillow Group Inc Class C (Z)	May 2019	28.23%
Insulet (PODD)	May 2019	26.98%
Zillow (ZG)	May 2019	26.85%
Calumet Specialty Products Partners (CLMT)	May 2019	24.63%
Enphase Energy (ENPH)	May 2019	23.33%
PagSeguro Digital (PAGS)	May 2019	22.31%
Encore Capital (ECPG)	May 2019	20.75%
Pampa Energia SA ADR (PAM)	May 2019	20.67%
Astronics (ATRO)	May 2019	20.15%
Grupo Financiero Galicia SA ADR (GGAL)	May 2019	19.34%
Powell Industries (POWL)	May 2019	17.76%
Cytokinetics (CYTK)	May 2019	17.04%
Torm PLC Class A (TRMD)	May 2019	16.12%
Telecom Argentina SA ADR (TEO)	May 2019	15.84%
Kinsale Capital (KNSL)	May 2019	15.75%
BBVA Banco Frances SA ADR (BBAR)	May 2019	15.56%

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