Two field and two greenhouse experiments were conducted to study the effects of environmental factors and genotype on leaf number in hybrid corn (Zea mays L.) and the relationship of leaf number to maturity. Data were secured on leaf number, leaf area, plant height, and the time required from planting to either silking or tasseling, depending upon the experiment. For 126 commercial hybrids grown in four counties in 1968, leaf number was significantly correlated with plant height, silking date and moisture at harvest. For the same hybrid, leaf number was only slightly (generally one leaf or less) if at all affected by seed source and growing conditions, either locations in the same year or between years. In this experiment, hybrids were grouped by reputed maturity and grown in a location appropriate to their maturity. Results were similar in 1969 when 95 of the 126 hybrids were grown at one location. Under greenhouse conditions, where temperatures were comparable and soil fertility, water stress, and pests controlled, days to tassel in 16 hybrids and leaf number were significantly correlated with maturity rating and with accumulated solar radiation. When grown during two seasons, spring and summer, a hybrid required approximately the same number of langleys of solar and sky radiation from planting to tasseling even though mean elapsed days varied significantly between seasons, for the 16 hybrids a mean of 64.6 days versus 48.8 days, respectively. Based upon the results of this experiment and reports of other researchers, it is suggested that leaf number could be a characteristic of potential use in rating corn hybrids for maturity.