The University of Wyoming balloonborne condensation nuclei (CN) record for 1973--1997 was analyzed to determine possible effects of commercial aircraft on the 8.6--12.7 km altitude range of the atmosphere, which includes the primary commercial airlanes between 29 and 41 kft. Thin layers of highly concentrated CN are often observed in this region of the atmosphere. Generally, aircraft flight information is not available for past balloon soundings making it impossible to ascribe a source to any specific observed CN layers. However, a CN layer observed in March 1997 was traced to a particular aircraft thus supporting the hypothesis that at least some of the observed layers are contrail remnants. Using the Laramie data set, a method was developed to quantify the enhancement in CN concentration induced by aircraft in comparison with natural background levels. We estimate conservatively that the contribution of the commercial aircraft fleet in the vicinity of Laramie, Wyoming, averaged over the period of record, amounts to about 10% of the background CN concentration. The frequency of occurrence of the CN layers approximately doubled from 1980 to 1992. ¿ 1998 American Geophysical Union