CONMED (CNMD) had the highest return between 1990 and 1991 by a small-cap US stock, returning 700%.

ASSET	YEARS	% RETURN
CONMED (CNMD)	1990-1991	700.01%
Mesa Laboratories (MLAB)	1990-1991	543.9%
BioLife Solutions (BLFS)	1990-1991	536.36%
National HealthCare (NHC)	1990-1991	461.28%
Digi International (DGII)	1990-1991	456.96%
Oppenheimer (OPY)	1990-1991	378.28%
STAAR Surgical (STAA)	1990-1991	317.86%
Cracker Barrel Old Country Store (CBRL)	1990-1991	281.11%
Matrix Service (MTRX)	1990-1991	213.85%
Lindsay (LNN)	1990-1991	185.69%
ODP (ODP)	1990-1991	180.56%
Axogen (AXGN)	1990-1991	178.13%
Transcontinental Realty Investors (TCI)	1990-1991	166.28%
Photronics (PLAB)	1990-1991	141.47%
Owens & Minor (OMI)	1990-1991	141.43%
CECO Environmental (CECO)	1990-1991	123.09%
Great Southern Bancorp (GSBC)	1990-1991	121.11%
Celldex Therapeutics (CLDX)	1990-1991	120%
Tootsie Roll Industries (TR)	1990-1991	116.79%
Heartland Express (HTLD)	1990-1991	112.13%
Bel Fuse A (BELFA)	1990-1991	112.11%
Universal (UVV)	1990-1991	109.77%
Astronics (ATRO)	1990-1991	100%
National Presto Industries (NPK)	1990-1991	90.01%
Marten Transport (MRTN)	1990-1991	87.55%

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