Background. Telephone helplines can facilitate referral, education and support for patients living with HIV or those concerned about the infection. The anonymity of helplines facilitates discussion of sensitive issues that are difficult to raise face to face. These services could support the expansion of HIV self-testing. However, maintaining quality and standardising messages in rapidly evolving fields such as HIV is challenging. Objectives. To evaluate the quality of the South African (SA) National AIDS Helpline. Methods. Mystery clients posing as members of the public made 200 calls to the service in 2014. They presented several scenarios, including having received HIV-positive results from a doctor's secretary or through self-testing. Following the call, 'clients' completed a semistructured questionnaire on the information received and the caller-counsellor interaction. Results. Calls were answered within a median of 5 seconds (interquartile range 2 -14). Conversations took place in 8 of the 11 SA official languages, though mainly in English. Overall, 75% of callers felt that with the information they received they could locate a nearby clinic for further services. Counsellors expressed appropriate levels of concern about inadequate counselling that callers had received and confidentiality breaches in some scenarios. Eight counsellors incorrectly mentioned the need for a waiting period to confirm a positive result. Consistent with policy, almost all said that being foreign would not affect HIV treatment access. About 90% explained the need for CD4+ testing and antiretroviral therapy, but only 78% discussed HIV prevention. Counsellors were mostly empathetic (83%), though some adopted a neutral tone (10%) or were brusque (6%) or unhelpful (2%). Conclusions. Overall, helpline counsellors were proficient at providing information about local clinics, HIV testing and steps needed for initiating ART. Dissatisfaction with the caller-counsellor interactions, instances of incorrect information and the relatively low attention accorded to HIV prevention are worrying, however. Training for both refreshing and updating knowledge, and supervision and monitoring of calls, could target these areas.