The effect of uncertainty in composite material properties on the nonlinear aeroelastic response and vibratory loads of a four-bladed composite helicopter rotor is studied. The aeroelastic analysis is done using a finite element method in space and time, and the composite cross section is analyzed using a variational asymptotic approach. The effective material properties of composite laminas are first considered as random variables with a coefficient of variation of 5%. The material uncertainty is propagated to cross-sectional stiffness,rotating natural frequencies, aeroelastic response, and vibratory loadsof the composite helicopter rotor. The stochastic cross-sectional and aeroelastic analyses are carried out with Monte Carlo simulations. The stochastic stiffness values are scattered up to 15% around the baseline stiffness values and show a Gaussian distribution with a coefficient of variation of about 4%. The uncertainty impact on rotating natural frequencies depends on the level of centrifugal stiffening for different modes. The stochastic rotating natural frequencies indicate a possibility of their coincidence with the integer multiples of rotor speed. The propagation of material uncertainty into aeroelastic response causes large deviations from the baseline predictions and affects the crucial higher harmonics content, which is critical for vibration predictions. The magnitudes of 4/rev vibratory loads show a scattering up to 300% from the baseline value,and their probability density functions show non-Gaussian-type distributions. Further, the uncertainty results for a coefficient of variation of 10% in the material properties are obtained. The uncertainty impact on the aeroelastic response is found to be proportional to the coefficient of variation of the composite material properties.