Our aim was to determine associated factors with non-invasive mechanical ventilation (NIMV) failure in acute hypercapnic respiratory failure ninety live patients treated with NIMV for acute hypercapnic respiratory failure were evaluated. While success of NIMV was defined as absence of need of intubation with the patient's discharge from hospital, failure of NIMV was defined as death or need of intubation. The pretreatment pH level was 7.30 in success and 7.28 in failure group (p> 0.05), PaCO(2) was 71.45 mmHg in success and 72.17 mmHg in failure group (p> 0.05). After 1h of NIMV, pH was 7.33 in success and 7.26 in failure group (p= 0.01), PaCO(2) was 65.50 mmHg in success and 73.47 mmHg in failure group (p= 0.02). After 1h of treatment, in success group there was significant increase of pH and decrease of PaCO(2) in contrast to baseline levels, while there was no significant change in failure group. The pretreatment Acute Physiology Assessment and Chronic Health Evaluation (APACHE) II score, serum C-reactive protein level and frequency of associated complication on admission were significantly higher and Glasgow Coma Score was lower in failure group. In conclusion, high APACHE II and C-reactive protein level, low Glasgow Coma Score, associated complication on admission and inadequate response in pH and PaCO(2) after first hour of NIMV are associated factors with NIMV failure.