Considerable research has been devoted to cotton boll development, but the relationship between leaf photosynthate production, allocation to fruiting structures, and boll maturation is unclear. Our research addresses these relationships with particular regard to the contributions of individual leaves to boll development within a representative fruiting branch. Main-stem leaves served a primary role in providing carbon for early fruiting branch development after which their contribution decreased. Studies of the fruiting branch showed carbon production by sympodial leaves was not well synchronized with the carbon requirements of subtended bolls. This was particularly true at the first two fruiting points along the branch where 85% of the total yield occurred. Although sympodial leaves were capable of supplying the majority of the carbon requirement of the boll, a decline in leaf photosynthesis occurred prior to an increase in demand from the subtended boll. Thus, for boll demand to be met, additional carbon must be imported from alternative sources. Possibly yield can be improved by maintaining leaf photosynthetic efficiency for a longer period of time.