We assessed isoniazid preventive therapy (IPT) completion and predictors among HIV-infected children and adults in two HIV clinics in Kinshasa, Democratic Republic of Congo. Between 1 September 2012 and 15 June 2013, 546 children (1-15 years) and 1532 adults (>15 years) were initiated on IPT; 86.6% (408/470) of the children and 88.2% (1129/1280) of the adults with an IPT outcome completed their therapy. Patients on antiretroviral therapy at IPT initiation were more likely to complete IPT.