Inventrust Properties (IVT) had the highest return in January 2015 by a mid-cap US stock, returning 57.5%.

ASSET	MONTH	% RETURN
Inventrust Properties (IVT)	January 2015	57.5%
Sibanye Gold Ltd ADR (SBSW)	January 2015	51.56%
DRDGOLD Limited ADR (DRD)	January 2015	49.68%
Halozyme Therapeutics (HALO)	January 2015	46.16%
Uniqure (QURE)	January 2015	40%
Ultragenyx (RARE)	January 2015	30.8%
NovaGold Resources (NG)	January 2015	30.42%
Endeavour Silver (EXK)	January 2015	29.86%
Ocular Therapeutix (OCUL)	January 2015	29.65%
SSR Mining (SSRM)	January 2015	27.75%
First Majestic Silver (AG)	January 2015	27.1%
Seabridge Gold (SA)	January 2015	25.78%
Alkermes (ALKS)	January 2015	23.36%
B2Gold (BTG)	January 2015	23.27%
Repligen (RGEN)	January 2015	22.49%
Catalyst Pharmaceuticals (CPRX)	January 2015	22.48%
Sandstorm Gold Ltd N (SAND)	January 2015	22.42%
CommScope (COMM)	January 2015	22.35%
Compania de Minas Buenaventura SAA ADR (BVN)	January 2015	21.49%
Stride (LRN)	January 2015	19.7%
Hecla Mining (HL)	January 2015	19.64%
Silicon Motion Technology (SIMO)	January 2015	16.52%
Vipshop (VIPS)	January 2015	16.25%
Synaptics (SYNA)	January 2015	15.9%
Credit Acceptance (CACC)	January 2015	15.38%

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