Amicus Therapeutics (FOLD) had the highest return in August 2014 by a mid-cap US stock, returning 71.6%.

ASSET	MONTH	% RETURN
Amicus Therapeutics (FOLD)	August 2014	71.6%
NexGen Energy (NXE)	August 2014	59.93%
Green Brick Partners (GRBK)	August 2014	43.14%
Catalyst Pharmaceuticals (CPRX)	August 2014	39.13%
Enphase Energy (ENPH)	August 2014	37.63%
Century Aluminum (CENX)	August 2014	33.51%
Trex (TREX)	August 2014	33.24%
American Eagle Outfitters (AEO)	August 2014	32.08%
RadNet (RDNT)	August 2014	30.47%
Autohome (ATHM)	August 2014	29.18%
Travere Therapeutics (TVTX)	August 2014	28.57%
OneMain (OMF)	August 2014	27.22%
Ocular Therapeutix (OCUL)	August 2014	26.79%
Delek Logistics Partners (DKL)	August 2014	25.79%
Compania de Minas Buenaventura SAA ADR (BVN)	August 2014	23.6%
Seabridge Gold (SA)	August 2014	22.52%
Ultragenyx (RARE)	August 2014	22.39%
Primoris Services (PRIM)	August 2014	21.86%
Avis Budget (CAR)	August 2014	20.58%
Installed Building Products (IBP)	August 2014	20.56%
Ambarella (AMBA)	August 2014	20.51%
Mueller Water Products (MWA)	August 2014	19.19%
MACOM Technology Solutions (MTSI)	August 2014	19.11%
Middleby (MIDD)	August 2014	18.53%
Mesoblast (MESO)	August 2014	18.49%

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