More than 100,000 mobile phone software applications ('apps') have been designed for the dissemination of health and medical information and healthcare and public health initiatives. This article presents a critical analysis of self-diagnosis smartphone apps directed at lay people that were available on the Apple App Store and Google Play in mid-April 2014. The objective of the analysis is to contribute to the sociology of diagnosis and to critical digital health studies by investigating the phenomenon of digitised diagnosis via apps. We adopted a perspective that views apps as sociocultural artefacts. Our analysis of self-diagnosis apps suggests that they inhabit a contested and ambiguous site of meaning and practice. We found that app developers combined claims to medical expertise in conjunction with appeals to algorithmic authority to promote their apps to potential users. While the developers also used appeals to patient engagement as part of their promotional efforts, these were undermined by routine disclaimers that users should seek medical advice to effect a diagnosis. More research is required to investigate how lay people are negotiating the use of these apps, the implications for privacy of their personal data and the possible effects on the doctor-patient relationship and medical authority in relation to diagnosis.