F5 Networks (FFIV) had the best month in 1999 by a large-cap US stock in June 1999, returning 300%.

ASSET	MONTH	% RETURN
F5 Networks (FFIV)	June 1999	300%
Suzano Papel e Celulose SA ADR (SUZ)	September 1999	287.48%
Lam Research (LRCX)	January 1999	111.7%
F5 Networks (FFIV)	October 1999	107.09%
Incyte (INCY)	December 1999	106.9%
VeriSign (VRSN)	December 1999	104.21%
Neurocrine Biosciences (NBIX)	December 1999	102.04%
Qualcomm (QCOM)	December 1999	93.81%
Cheniere Energy (LNG)	January 1999	92.04%
Skyworks Solutions (SWKS)	April 1999	91.84%
Cognizant Technology Solutions Corp Class A (CTSH)	December 1999	91.17%
Sterling Construction (STRL)	January 1999	90.91%
Neurocrine Biosciences (NBIX)	November 1999	89.29%
Onto Innovation (ONTO)	November 1999	83.67%
Agilent Technologies (A)	December 1999	83.26%
ViaSat (VSAT)	December 1999	81.36%
Arrowhead Pharmaceuticals (ARWR)	October 1999	80%
ArcelorMittal SA ADR (MT)	April 1999	78.63%
SK Telecom Co Ltd ADR (SKM)	November 1999	77.83%
Booking (BKNG)	April 1999	76.98%
CoStar (CSGP)	January 1999	75.7%
Copart (CPRT)	December 1999	71.7%
Qualcomm (QCOM)	March 1999	70.96%
MicroStrategy (MSTR)	October 1999	70.08%
Lam Research (LRCX)	June 1999	69.75%

Large-cap stocks are treated as having a market cap of $10B+.