We present the design and performance of a temperature‐ and pressure‐controlled sample cell for the small‐angle x‐ray diffraction analysis of lyotropic liquid crystalline materials. The particular requirements for such a system of high resolution, high intensity, and control on sample homogeneity can be particularly well fulfilled using line‐source diffraction. We thus designed a sample cell that will allow the use of a line source. Sample volume is kept small (20 &mgr;l) to keep the contained energy low. The hydrostatic system, based on beryllium x‐ray windows, Bridgman type seals with lead O rings and a stainless‐steel body is rated to achieve pressure control within ±3 bar between 0 and 3000 bar at ramping rates of 30–100 bar/s. Temperature control is achieved within ±0.03 K between room temperature and 180 °C. Maximum heating rates correspond to 0.8 K/s. Control algorithms are implemented on a Microrobotics K4 controller, using a modest amount of computing power. ©1996 American Institute of Physics.