The performance of the GOSSYM-COMAX simulation model and expert system in a cotton growing region will dictate the usage and confidence in using the model as a cotton management tool. This study was conducted to evaluate the performance of GOSSYM-COMAX in Missouri from 1987 to 1991. Data files were collected from 57 fields that were simulated by either research/extension personnel or farmers during the 5 year period. The agreement between the actual and simulated fields was generally good, with the agreement indices (AI) ranging from 0.71 to 0.77 for growth and yield parameters. During bloom period, an AI of 0.89 was found for leaf N level, while the AI for evapotranspiration (ET), was 0.96, indicating that GOSSYM was able to simulate the nitrogen balance and water use aspects of crop growth to support its use as a management tool, when used in concert with other management tools.