This note deals with the problem of stability analysis for linear systems with uncertain real, possibly time-varying, parameters. A robust stability approach based on a Lyapunov function which depends quadratically on the uncertain parameters as well as in the system state is proposed. This robust stability approach, referred to as biquadratic stability, is suited to deal with uncertain real parameters with magnitude and rate of change which are confined to a given convex region. A linear matrix inequality (LMI) based sufficient condition for biquadratic stability is developed. The proposed robust stability analysis method includes quadratic stability and affine quadratic stability as particular cases.