Because myotonic dystrophy patients show marked irregularities of breathing both awake and asleep, variables related to breathing pattern under both conditions were measured in 11 patients, together with pulmonary function indices, ventilatory CO2 response and maximal mouth pressures. The aim of the study was to detect and explain a possible interrelationship between daytime and nocturnal irregularity. Awake, patients demonstrated significantly more variability in tidal volume and respiratory cycle time than controls. Asleep, periodic breathing occurred during up to 100% of the time spent in light sleep, but not during deep sleep. A strong correlation was found with age (r = 0.73, p = 0.01). No relationship was found between disturbed breathing awake and asleep. There was a tendency for increased variability of tidal volume awake in cases with a decreased ventilatory CO2 response (p = 0.1). The results indicate that different mechanisms may be involved in daytime and nocturnal irregularity. It is hypothesized that brain stem integrative functions may be impaired in myotonic dystrophy.