We present data for single and binary massive stars, both from new analyses (Trapezium stars, O stars in NGC 6611, and the massive binary system V478 Cyg) and collected from the literature. We show that data for single stars depart from the theoretical ZAMS, while those for binary systems are consistent with a ZAMS formed through a linearly mass-dependent accretion rate. In spite of this difference, spectroscopic and dynamical analyses give consistent results in several binary systems.