Idaho Strategic Resources (IDR) had the highest return in December 2015 by a small-cap US stock, returning 125%.

ASSET	MONTH	% RETURN
Idaho Strategic Resources (IDR)	December 2015	125%
LightPath Technologies (LPTH)	December 2015	62.64%
Perion Network (PERI)	December 2015	48.16%
Collegium Pharmaceutical (COLL)	December 2015	43.98%
Westlake Chemical Partners (WLKP)	December 2015	42%
Cadiz (CDZI)	December 2015	41.02%
American Superconductor (AMSC)	December 2015	33.82%
Five9 (FIVN)	December 2015	32.22%
Semler Scientific (SMLR)	December 2015	28.64%
Pacific Biosciences of California (PACB)	December 2015	28.1%
Newegg Commerce (NEGG)	December 2015	27.68%
Ur Energy (URG)	December 2015	25%
Canadian Solar (CSIQ)	December 2015	24.56%
MGP Ingredients (MGPI)	December 2015	24.28%
Eyepoint Pharmaceuticals (EYPT)	December 2015	24.23%
G Willi-Food International (WILC)	December 2015	23.01%
McEwen Mining (MUX)	December 2015	21.7%
PowerFleet (AIOT)	December 2015	21.05%
Transcontinental Realty Investors (TCI)	December 2015	20.61%
Unity Bancorp (UNTY)	December 2015	20.11%
Ballard Power Systems (BLDP)	December 2015	20%
Cementos Pacasmayo SAA ADR (CPAC)	December 2015	19.72%
Akebia Ther (AKBA)	December 2015	19.52%
Smith & Wesson Brands (SWBI)	December 2015	19%
Civista Bancshares (CIVB)	December 2015	18.58%

Small-cap stocks are treated as having a market cap between $300M and $2B.