Background: Diffuse myocardial fibrosis can be quantified by calculating extra-cellular volume (ECV) from native and post-contrast T1 values using dedicated single bolus contrast medium injection protocols. Purpose: To evaluate differences in T1 maps and myocardial ECV measurements in routine stress/rest perfusion cardiovascular magnetic resonance (CMR) examinations after injection of single and double dose of contrast medium. Material and Methods: Thirty-seven consecutive patients (30 men; mean age, 62 +/- 13 years) underwent clinically indicated adenosine stress/rest perfusion CMR examination to rule out myocardial ischemia following a conventional split-dose contrast medium injection strategy. Native and post-contrast T1 mapping was performed 15 min after the first (0.1 mmol/kg) and second (0.1 mmol/kg) dose of contrast medium using a breath-held Modified Look-Locker Inversion recovery (MOLLI) sequence. Student's t-test for paired samples, Bland-Altman plots, and concordance-correlation coefficients (CCC) for agreement between T1 and ECV calculations after single and double dose of contrast medium were calculated. Intra- and inter-observer agreement for measurements was also analyzed. Results: Myocardial T1 values after single and double dose of contrast medium significantly differed (mean difference of 114.1 +/- 19.9 ms, P< 0.01). A single dose of contrast agent provided slightly higher ECV values (mean difference of 2.3 +/- 1.1%). CCC for ECV calculations was 0.66. Intra- and inter-observer agreement for all measurements was excellent (CCC >= 0.83). Conclusion: Quantification of myocardial ECV on conventional stress/rest perfusion CMR examination is feasible. T1 maps obtained 15 min after 0.1 mmol/kg of contrast medium provide slightly higher myocardial T1 measurements and ECV values compared with T1 maps obtained after a total dose of 0.2 mmol/kg.