Weatherford International (WFRD) had the highest return in May 2019 by a mid-cap US stock or mid-cap US stock, returning 11,300%.

ASSET	MONTH	% RETURN
Weatherford International (WFRD)	May 2019	11,300%
Hut 8 Corp Common Stock (HUT)	May 2019	50.8%
Soleno Therapeutics (SLNO)	May 2019	43.9%
TransMedics (TMDX)	May 2019	34.27%
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%
Avantor (AVTR)	May 2019	25%
Quantum Computing (QUBT)	May 2019	25%
Enphase Energy (ENPH)	May 2019	23.33%
PagSeguro Digital (PAGS)	May 2019	22.31%
Pampa Energia SA ADR (PAM)	May 2019	20.67%
Goosehead Insurance (GSHD)	May 2019	20.22%
Companhia Siderurgica Nacional ADR (SID)	May 2019	19.46%
Grupo Financiero Galicia SA ADR (GGAL)	May 2019	19.34%
SolarEdge Technologies (SEDG)	May 2019	19.33%
United Parks & Resorts (PRKS)	May 2019	19.14%
Powell Industries (POWL)	May 2019	17.76%
Cytokinetics (CYTK)	May 2019	17.04%
Companhia Paranaense de Energia Pref ADR (ELP)	May 2019	16.73%
Guardant Health (GH)	May 2019	16.52%
Torm PLC Class A (TRMD)	May 2019	16.12%
Telecom Argentina SA ADR (TEO)	May 2019	15.84%
Palomar (PLMR)	May 2019	15.19%

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