Cytokinetics (CYTK) had the highest return in January 2013 by a mid-cap US stock, returning 76.1%.

ASSET	MONTH	% RETURN
Cytokinetics (CYTK)	January 2013	76.12%
Globalstar, Inc Common Stock (GSAT)	January 2013	72.5%
Corcept Therapeutics (CORT)	January 2013	42.76%
Ionis Pharmaceuticals (IONS)	January 2013	36.69%
Telecom Argentina SA ADR (TEO)	January 2013	35.18%
Amicus Therapeutics (FOLD)	January 2013	34.75%
Nexstar Broadcasting (NXST)	January 2013	31.68%
Bright Horizons Family Solutions (BFAM)	January 2013	27.27%
Walker & Dunlop (WD)	January 2013	27.25%
Oshkosh (OSK)	January 2013	26.88%
Popular (BPOP)	January 2013	26.37%
Icahn Enterprises (IEP)	January 2013	26.3%
ACADIA Pharmaceuticals (ACAD)	January 2013	26.15%
Primoris Services (PRIM)	January 2013	24.71%
Transocean (RIG)	January 2013	24.64%
Assured Guaranty (AGO)	January 2013	24.6%
Pampa Energia SA ADR (PAM)	January 2013	23.85%
WNS (WNS)	January 2013	22.05%
Sharplink Gaming (SBET)	January 2013	21.41%
Alkermes (ALKS)	January 2013	21.37%
Melco Resorts & Entertainment (MLCO)	January 2013	21.02%
MakeMyTrip (MMYT)	January 2013	20.92%
Madrigal Pharmaceuticals (MDGL)	January 2013	20.82%
H&R Block (HRB)	January 2013	20.73%
AutoNation (AN)	January 2013	20.65%

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