The flow properties of organic powders are of great significance for the transfer, sampling and mixing of materials in the fine chemical, pharmaceutical, cosmetic, food, and agricultural fields. The following studies were performed to describe the complex flow behavior of lactose powders, a common pharmaceutical excipient. Pharmaceutical powder flow is a crucial element of successful dosage form design and manufacture. Three lactose samples, bulk (median diameter=52.3 mu m, sigma(g)=1.9), 45-75 mu m and 125-180 mu m, were studied. Their flow patterns were analyzed and irregular oscillations of the dynamic angle of repose around the mean were observed. Broad band frequency distributions were observed in the power spectra. Phase-space plots revealed complex non-overlapping trajectories. Return maps exhibited grouping of data points with a well defined structure indicative of a chaotic system. The combined observations are consistent with underlying order, and were observed in all lactose samples. These studies are among the first demonstrating chaos in organic powders. It is anticipated that powder specific chaotic inputs may be used to manipulate and improve powder flow at various stages in the pharmaceutical manufacturing process.