Weatherford International (WFRD) had the highest return in December 2019 by a mid-cap US stock, returning 120,172.8%.

ASSET	MONTH	% RETURN
Weatherford International (WFRD)	December 2019	120,172.81%
Mirum Pharmaceuticals (MIRM)	December 2019	235.43%
Axsome Therapeutics (AXSM)	December 2019	156.41%
Immunitybio (IBRX)	December 2019	115.34%
BW LPG (BWLP)	December 2019	87.47%
Soleno Therapeutics (SLNO)	December 2019	80.37%
Antero Midstream Partners (AM)	December 2019	64.28%
Cidara Therapeutics (CDTX)	December 2019	62.03%
Power Solutions International, Inc Common Stock (PSIX)	December 2019	56.44%
Scholar Rock (SRRK)	December 2019	55.24%
Immunovant (IMVT)	December 2019	52.6%
Kinetik (KNTK)	December 2019	46.67%
Sitime (SITM)	December 2019	45.71%
Verona Pharma PLC ADR (VRNA)	December 2019	44.11%
Banco Macro SA B ADR (BMA)	December 2019	39.37%
Corporacion America Airports (CAAP)	December 2019	37.93%
Transocean (RIG)	December 2019	37.33%
Hecla Mining (HL)	December 2019	37.25%
Vista Oil Gas ADR (VIST)	December 2019	36.52%
Alpha Metallurgical Resources (AMR)	December 2019	35.68%
TG Therapeutics (TGTX)	December 2019	35.2%
SM Energy (SM)	December 2019	33.18%
Civitas Resources (CIVI)	December 2019	32.54%
Acm Research (ACMR)	December 2019	31.78%
Solaris Energy Infrastructure (SEI)	December 2019	30.67%

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