An experimental study on void fraction in a small diameter tube (9.525 mm i.d.) for vertical upward and downward, gas-liquid, two-phase flow was carried out. Void fraction measurements in water-air were taken for both upward and downward flow using a single beam gamma densitometer operated in the count mode. Based on these data, new correlations for predicting the void fraction in vertical, gas-liquid, upward and downward two-phase flow were developed using a dimensional analysis approach. A new parameter z, as a function of the liquid Reynolds number, the gas Reynolds number, and the Froude number of the gas phase, was introduced. This parameter indicates the amount of the gas mass velocity that contributes to the increase in the void fraction. The higher this parameter is, the larger will be the increase in the void fraction. It was also found that the weighting parameter, z, has different values for different flow regimes, and is therefore more applicable to two-phase flow modeling and analysis. A downward void fraction correlation, in terms of the upward void fraction, was also developed such that it incorporates the effects of changing the flow orientation (with respect to gravity). It was shown from the two correlations that downward void fraction values are usually higher than those in upward flow. For lower liquid Reynolds numbers (Re(L) < 17400) the difference between the upward and downward void fraction was much higher than that in the case for Re(L) > 17400. At the highest liquid Reynolds numbers (e.g., Re(L) = 74000), such difference was almost neglible (as small as 5%). Comparisons were made between the newly developed correlations for upward and downward flow and some of the available experimental data and correlations in the literature. Excellent agreements were obtained in all cases.