Mepiquat Chloride (MC) is the most common plant growth regulator used in commercial cotton production. MC allows producers to regulate vegetative growth to match current weather conditions. This study proposes the use of the Average Length Technique (of the uppermost five internodes), ALT5 as an indicator of potential plant height. The information can be used to determine the need for applications of MC. Objectives of this study are to (1) present assumptions made in the design and calibration of the technique and (2) to demonstrate the use of ALT5 to forecast final plant height at the cessation of vegetative growth (cutout). Data used to design ALT5 were collected during 1991 at Corpus Christi, Texas, using cultivar DPL-50. In 1995, a second experiment was conducted to validate the effectiveness of ALT5 to forecast final plant height. Experimental treatments were (1) untreated control, (2) MC applications made to maintain ALT5 below 1.6 inches and (3) MC applied to maintain ALT5 below 1.4 inches. Final plant height was measured at harvest in all the treatments. ALT5 is based on the following two assumptions: (1) individual internodes attain their maximum length in a period of 12 to 15 days from their initiation and (2) the time course of plant height development follows a sigmoidal growth pattern. A color-coded ruler or stick was developed to facilitate the measurement and estimation of ALT5. The ruler was calibrated to relate ALT5 measurements to potential plant height at the cessation of vegetative development. Initial evaluation and validation of this technique has proven satisfactory. The technique accurately predicted final plant height in an experiment conducted at Corpus Christi, TX, in 1995. The research showed that ALT5 measurements are more sensitive to changes in growth rates induced by the application of MC than using the height-to-node ratio for the entire plant. The data also suggest that ALT5 measurements may be used to quantify the effects of water and nutrient stress on main stem elongation rate.