Polygamy is the practice of marriage to multiple partners. Approximately 6-11% of households in Uganda and 4-11% of households in Kenya are polygamous. The complex families produced by polygamous marriage customs give rise to additional considerations for healthcare providers and public health messaging around HIV care. Using 27 in-depth, semi-structured qualitative interviews with participants in two studies in rural Kenya and Uganda, we analysed challenges and opportunities that polygamous families presented in the diagnosis, treatment and prevention of HIV, and provider roles in improving HIV outcomes in these families. Overall, prevention methods seemed more justifiable to families where co-wives live far apart than when all members live in the same household. In treatment, diagnosis of one member did not always lead to disclosure to other members, creating an adverse home environment; but sometimes diagnosis of one wife led not only to diagnosis of the other, but also to greater household support.