A flow pattern specific model is presented, which is used to estimate the overall two-phase pressure drop for vertical upflow. The model first predicts the flow pattern that is present. For bubbly flow a drift flux model is employed, slug flow is dealt with by a mechanistic description invoking separate section around and between the Taylor bubbles, For annular flow, a phenomenological approach is used, utilizing published descriptions for the rates of entrainment and deposition and film thickness, but employing a new, improved correlation for interfacial friction factor. It was found that the same model gave the best description of churn flow. The model has been tested against data from small diameter circular and noncircular channels obtained from experiments described in the paper, as well as published experimental data. The agreement is good over all flow patterns.