With field observations in situ, and extended study of recently collected copious FAA-conserved material of Pentastemona sumatrana, it became evident that Pentastemona represents a family of its own, Pentastemonaceae, beside Stemonaceae. The discriminating characters are discussed and additional notes on the growth habit, inflorescences, and flowers of Pentastemona, which appear possibly to be polygamous, are given. For the genus Stemona two new combinations are proposed, and some notes on cultivated living plants are presented.