Sawlogs are in short supply in northern Ontario, and thinning has been suggested as one way to improve the situation. The only rotation-age jack pine (PinusbanksianaLamb.) thinning trial in the region was examined to assess how commercial thinning influenced wood quality. This report covers an unreplicated trial of a 65-year chronology of pith to bark relative densities and growth rates based on X-ray densitometry of breast-height increment cores taken from trees on two thinned plots (average spacing 2.6 and 3.4 m) and an unthinned control (average spacing 1.7 m). The trees on the treatment plots responded to thinning by producing wood with significantly lower relative density than those on the control plot. This trend continued much longer than reported for other pines and could negatively affect pulp yield or mechanical properties of lumber. Enhanced earlywood growth caused a drop in the proportion of latewood that resulted in the decline in density. Thinning may have improved moisture availability during the early and middle season and encouraged earlywood growth. Density and growth rate differences became apparent soon after treatment. Early, rapid, and inexpensive estimates of the product potential of younger thinning trials are possible using the techniques demonstrated here.