A comparison was made of temperature and wind observations reported by rawinsonde and Aircraft Communications, Addressing, and Reporting System (ACARS)-equipped commercial aircraft separated by less than 150 km in distance and 90 min in time near Denver, Colorado, in February and March 1992. Only data made on aircraft ascents and descents reported automatically were used. A total of 4440 matched data pairs were obtained for this period. The sample was analyzed in total but also as a function of time and distance separation, height, time of day, and ascent versus descent. A standard deviation temperature difference of 0.97 degrees C and rms vector wind difference of 5.76 m s(-1) were found for the entire sample but were reduced, respectively, to 0.59 degrees C and 4.00 m s(-1) when only data pairs separated by less than 25 km and 15 min were used. The study provides an upper limit to the combination of rawinsonde and ACARS observation and reporting errors and mesoscale variability, but it is not possible to distinguish the exact contributions from each of these sources. However, overall, these statistics indicate that the rawinsonde data used were more accurate than that reported in a previous study and that the accuracy of the ACARS data was somewhat higher still.