The effect of altitude on lignification is important in developing an understanding of what drives natural variation in wood properties. Cambia of two Norway spruce trees, growing at altitudes of 580 and 1260 m a.s.l., were periodically sampled to measure tracheid dimensions and cellular lignin content. The low-elevation tree showed a higher rate of cell division with thicker cell walls and wider growth rings. The maturation phase of tracheids at high elevation was completed by the end of October while low elevation latewood tracheids were still lignifying. As revealed by ultraviolet microscopy, lignin content in single cells, as well as in complete tree-rings, was higher at high elevation. Rank correlation analysis indicated high negative correlation between lignin content of tracheid cell walls and corresponding wall thickness. It is hypothesized that trees growing at higher altitudes compensate for the thinner cell walls with an increased lignin content which helps to maintain mechanical integrity of the xylem.Key words: altitude, lignin, tracheid, wood formation, ultraviolet microscopy.