Nocturnal muscle activity in legs (MAL) was studied in 19 patients with sleep apnea syndrome before and during nCPAP. We distinguished periodic (P-MAL) and nonperiodic (N-MAL) as well as apnea-associated and independent MALs. N-MALs were strongly associated with apneas and disappeared during nasal continuous positive airway pressure (nCPAP). P-MALs showed a rather long intermovement interval (IMI) of about 54 s when associated with respiratory disturbances and a shorter one of about 38 s when occurring independently. In 5 patients with predominantly independent P-MALs and a short IMI before and during nCPAP an incidental coexistence of sleep apnea with periodic leg movements in sleep syndrome (PLMS) was postulated. Five patients with an equal number of apnea-associated and independent MALs before nCPAP had a long IMI which shortened under nCPAP, while the majority of MALs become independent. This could indicate a facilitating or an unmasking effect of nCPAP on PLMS. Four patients with predominantly apnea-associated MALs with a long IMI before nCPAP showed an unchanged MAL pattern under nCPAP, despite the disappearance of apneas. Since some arousals persisted in these patients, insufficient nCPAP pressure with an upper airway resistance syndrome was suspected. We conclude that when P-MALs persist under nCPAP therapy a long IMI may indicate an upper airway resistance syndrome, whereas a short IMI may indicate a coexisting or even induced PLMS.