Phosphoinositide 3-kinases produce 3′-phosphorylated phosphoinositides that act as second messengers to recruit other signalling proteins to the membrane 1. Pi3ks are activated by many extracellular stimuli and have been implicated in a variety of cellular responses 1. The Pi3k gene family is complex and the physiological roles of different classes and isoforms are not clear. The gene Pik3r1 encodes three proteins (p85α, p55α and p50α) that serve as regulatory subunits of class I A Pi3ks (ref. 2). Mice lacking only the p85α isoform are viable but display hypoglycaemia and increased insulin sensitivity correlating with upregulation of the p55α and p50α variants 3. Here we report that loss of all protein products of Pik3r1 results in perinatal lethality. We observed, among other abnormalities, extensive hepatocyte necrosis and chylous ascites. We also noted enlarged skeletal muscle fibres, brown fat necrosis and calcification of cardiac tissue. In liver and muscle, loss of the major regulatory isoform caused a great decrease in expression and activity of class I A Pi3k catalytic subunits; nevertheless, homozygous mice still displayed hypoglycaemia, lower insulin levels and increased glucose tolerance. Our findings reveal that p55α and/or p50α are required for survival, but not for development of hypoglycaemia, in mice lacking p85α.