Normalized Lyα flux R_Lyα = f_Lyα/f_bol as a function of normalized X-ray flux R_X = f_X/f_bol for all the IUE (⋄) and HST (•) Lyα flux measurements of M dwarfs in the literature. Our measurement for GJ 436 data is shown in blue. The dotted lines link HST and IUE mesasurements for objects that have been measured with both instruments. The plain line show the best-fit linear regression to HST measurements, log (R_Lyα) ≈ 0.5log (R_X) − 2.2. The grey dashed line indicates the log R_Lyα = log R_X relation. The horizontal error bars in the X-ray flux are the dispersions reported in Table 2.