Globalstar, Inc Common Stock (GSAT) had the highest return in December 2018 by a mid-cap US stock, returning 56.1%.

ASSET	MONTH	% RETURN
Globalstar, Inc Common Stock (GSAT)	December 2018	56.1%
Perpetua Resources (PPTA)	December 2018	50.05%
CleanSpark (CLSK)	December 2018	35.76%
Kiniksa Pharmaceuticals (KNSA)	December 2018	33.63%
ADEIA (ADEA)	December 2018	28.07%
Energy of Minas Gerais Co DRC (CIG.C)	December 2018	22.07%
Americas Silver (USAS)	December 2018	21.32%
B2Gold (BTG)	December 2018	16.34%
Zai Lab (ZLAB)	December 2018	16.22%
Osisko Gold Ro (OR)	December 2018	15.87%
Frontdoor (FTDR)	December 2018	13.72%
Merus (MRUS)	December 2018	12.67%
SSR Mining (SSRM)	December 2018	12.57%
Compania de Minas Buenaventura SAA ADR (BVN)	December 2018	12.48%
Endeavour Silver (EXK)	December 2018	11.98%
Seabridge Gold (SA)	December 2018	11.18%
Fortuna Silver Mines (FSM)	December 2018	10.64%
Grupo Aeroportuario del Sureste SAB de CV ADR (ASR)	December 2018	10.48%
Burford Capital (BUR)	December 2018	9.75%
Companhia Energetica de Minas Gerais CEMIG Pref ADR (CIG)	December 2018	9.04%
Grupo Simec SAB de CV ADR (SIM)	December 2018	8.35%
Match (MTCH)	December 2018	8.26%
Indivior PLC Ordinary Shares (INDV)	December 2018	8%
Denali Therapeutics (DNLI)	December 2018	7.77%
Ultrapar Participacoes SA ADR (UGP)	December 2018	7.29%

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