Specimens of carbromal obtained by crystallization from various solvents were analyzed. After a 9-month storage period, the occurrence of three polymorphic forms (I, II and III) was ascertained. The metastable form III obtained by crystallization from dimethylformamide showed a reasonably high stability. Preliminary pharmacological tests revealed clear-cut differences in the therapeutic effect between a commercial specimen and the polymorphic modification III. The results presented here indicate the importance of polymorphism of carbromal for medicinal use.