Successful cotton harvest practices are largely dependent upon the use of harvest aid chemicals. However, the performance of harvest aids is often inconsistent making results unpredictable. Weather before, at the time of, and after application of a chemical can greatly influence its efficacy. The objective of this study was to determine which weather factors have the greatest influence on cotton's responses to Folex (1.26 kg hg-1 tribufos) and Folex/Prep (0.63 kg hg-1 tribufos and 1.12 kg hg-1 ethephon) harvest aids. A five-year (1992 - 1996) study was conducted at 16 locations (4 in TX, 1 each in OK, AR, LA, MS, MO, TN, AL, GA, NC, SC, FL and CA) throughout the U.S. Cotton Belt. Harvest aid response data collected in the field were performance (aggregate evaluation of defoliation, desiccation, and boll opening responses, 0-100 scale) at 7 and 14 DAT; percent defoliation, dessication and boll opening at 7 and 14 DAT; and percent top and bottom regrowth at 21 DAT. Weather data collected were average daily maximum and minimum temperatures, DD60 and rainfall from planting to treatment and from treatment to 14 DAT; cloud cover, air temperature, relative humidity, and wind speed at time of treatment; and rainfall 7 days prior and 7 days after treatment. Correlation analysis and mean difference tests (p<.01) were used to determine the effects of weather on harvest aid responses. Results showed that the performance of and defoliation with both Folex and Folex/Prep improved when the average minimum temperature from planting to application increased from 18C to 20C. This seems to be a small increase in average minimum temperature, so its effect on performance and defoliation is remarkable. Desiccation was not affected by weather in this study. At average maximum temperatures below 26C, Prep had a greater relative influence on boll opening that at temperatures above 31C. A similar trend was observed with minimum temperatures and with DD60 accumulation. This suggests that Prep can produce a beneficial boll opening response at lower temperatures. Results also suggest that regrowth was less prevalent when these harvest aids were applied in relatively cool, wet environments. Perhaps the most significant finding is that the response of cotton to harvest aid chemicals in this study appeared to be more affected by weather factors during the growing season and around the time of treatment over which the producer has little control, rather than by those weather factors at the exact time of treatment, over which the producer has some limited control.