Seasonal variations in structural parameters were calculated from Falling Weight Deflectometer data on 25 LTPP flexible seasonal monitoring sections. In all, 23,976 deflection basins were analyzed. The deflection basins were collected monthly between the fall of 1993 and the spring of 1995. The test locations were distributed around North America from Saskatchewan to South Texas and from Idaho to Maine. The sections all consisted of asphalt surfacing and granular bases (and some granular subbases) over subgrades. Asphalt thicknesses ranged from 46 to 277#mm. Seasonal variations observed on selected sections in this database and documented in this paper include variations in asphaltic materials with temperature, variations in moduli of the unbound materials with precipitation, and variations in moduli of unbound materials due to freeze/thaw. For moderately thick to thick asphalt sections (>125 mm) the temperature dependency can clearly be seen in the deflection basins. As expected, this variation in deflections, and AC modulus is sinusoidal over the year. The amplitude of the sinusoidal variations were found to be heavily influenced by the thickness of the AC layer. Thicker AC sections exhibited higher sinusoidal variations. Base and subgrade values tended to be influenced by temperature but remained more consistent throughout the year except in cases where freeze?thaw effects were seen. During freeze conditions, the moduli of the unbound (base and subgrade) layers drastically increased.