The structure of sleep and number of body movements (BMS) and periodic leg movements during sleep (PMS), were studied in three unrelated girls suffering from L-DOPA responsive hereditary dystonia with marked diurnal fluctuation and in their 11 healthy, close relatives. All three girls had an increased number of BMS during rapid eye movement (REM) sleep. Five of the six parents and three siblings had abnormal PMS. One pair of parents had BMS similar to those of their affected daughter. The occurrence of BMS and PMS in the families studied may indicate a common mechanism for both. Because familial PMS is quite rare in its pure form, and this type of dystonia is also rarely encountered, the occurrence of BMS and PMS in members of these families may imply a causative relation between these two sleep-related motor phenomena.