A detailed statistical analysis of monthly averages of ozonesonde readings is performed to assess trends in ozone in the troposphere and the lower to mid-stratosphere. Regression time series models, which include seasonal and trend factors, are estimated for 13 stations located mainly in the mid-latitudes of the northern hemisphere. At each station, trend estimates are calculated for 14 ''fractional'' Umkehr layers covering the altitude range from 0 to 33 km. For the 1970--1982 period, our main findings indicate an overall negative trend in ozonesonde data in the lower stratosphere (15--21 km) of about -0.5% per year, and some evidence of a positive trend in the troposphere (0--5 km) of about 0.8% per year. An in-depth sensitivity study of the trend estimates is performed with respect to various correction procedures used to normalize ozonesonde readings to Dobson total ozone measurements. The main results indicate that the negative trend findings in the 15 to 21-km altitude region are robust to the normalization procedures considered.