To study the impact that heated humidification instituted in the beginning of automatic positive airway pressure (APAP) therapy has on compliance with and the side effects of the treatment. Thirty-nine treatment-naïve patients with obstructive sleep apnea were randomized into two groups to receive APAP using one of two modalities: with heated humidification (APAPwith group); and without heated humidification (APAPw/o group). Patients were evaluated at 7 and 30 days after APAP initiation. The following parameters were analyzed: compliance with treatment (mean number of hours/night); side effects (dry nose or mouth, nasal obstruction and rhinorrhea); daytime sleepiness (Epworth sleepiness scale score) and subjective comfort (visual analog scale score). Patients were also evaluated in terms of residual apnea-hypopnea index (AHI), as well as mean pressures and leaks registered in the ventilators. There were no differences between the two groups in terms of mean age (APAPwith: 57.4±9.2; APAPw/o: 56.5±10.7 years), AHI (APAPwith: 28.1±14.0; APAPw/o: 28.8±20.5 events/hour of sleep), baseline Epworth score (APAPwith: 11.2±5.8; APAPw/o: 11.9±6.3) and initial nasal symptoms. Compliance was similar in both groups (APAPwith: 5.3±2.4; APAPw/o: 5.2±2.3 h/night). There were no differences in any of the other parameters analyzed. The introduction of heated humidification at the beginning of APAP therapy provided no advantage in terms of treatment compliance or side effects of treatment.