Phosphatidylinositol 3-phosphate [PtdIns(3)P] acts as a second messenger via the recruitment of diverse signalling proteins to various cellular compartments. Recent advances have highlighted the association of human diseases with mutations in phosphatases that regulate PtdIns(3)P levels. Myotubularin, the gene mutated in myotubular myopathy, functions as a lipid phosphatase with specificity for PtdIns(3)P. It is now apparent that there is an increasing family of proteins that are defined by their significant homology with myotubularin. The myotubularin-related gene family includes proteins that exhibit a lipid phosphatase catalytic motif, those that contain mutations of the critical catalytic residues, and at least one protein that functions as an adapter subunit for PtdIns(3)P-3-phosphatase activity. The present challenge is to understand how deregulation of phosphoinositide metabolism causes human disease.