A five-year time series of lidar profiles from a site in Colorado is used to investigate the sources and scales of variability for ozone in the free troposphere. Analysis of 475 daily mean profiles derived from more than 25,000 individual profiles shows that ~66% of the total variance is due to fluctuations on time scales from 1 day to 1 month, ~30% on time scales from 1 month to 1 year, and ~4% on time scales greater than 1 year. The analysis also identifies low-frequency fluctuations correlated with the El Ni¿o/Southern Oscillation (ENSO) (following a ~6-month lag) that may account for as much as 5% of the total variance in the data. The ENSO-related changes can also lead to apparent ozone trends that range from -0.2 to +0.5 ppbv/yr for different 10-year intervals between 1970 and 1990. ¿ 1998 American Geophysical Union