The acceptability of lifelong antiretroviral therapy (ART) among HIV-positive women in high-burden Nigeria, is not well-known. We explored readiness of users and providers of prevention of mother-to-child transmission of HIV (PMTCT) services to accept lifelong ART -before Option B plus was implemented in Nigeria. We conducted 142 key informant interviews among 100 PMTCT users (25 pregnant-newly-diagnosed, 26 pregnant-in-care, 28 lost-to-follow-up (LTFU) and 21 postpartum women living with HIV) and 42 PMTCT providers in rural North-Central Nigeria. Qualitative data were manually analyzed via Grounded Theory. PMTCT users had mixed views about lifelong ART, strongly influenced by motivation to prevent infant HIV and by presence or absence of maternal illness. Newly-diagnosed women were most enthusiastic about lifelong ART, however postpartum and LTFU women expressed conditionalities for acceptance and adherence, including minimal ART side effects and potentially serious maternal illness. Providers corroborated user findings, identifying the postpartum period as problematic for lifelong ART acceptability/adherence. Option B plus scale-up in Nigeria will require proactively addressing PMTCT user fears about ART side effects, and continuous education on long-term maternal and infant benefits. Structural barriers such as the availability of trained providers, long clinic wait times and patient access to ART should also be addressed.