Both physiological and self-measurement methods have been employed to collect data on the dimensions of the erect penis. However, self-measurement using paper strips has often been favored as a less intrusive and time-consuming method, despite the recognition of the increased chance of bias through exaggeration. The current study aimed to establish the test-retest reliability of measurement of the erect penis using paper strips in a sample of 312 gay men. The men were issued with color-coded measuring strips printed with instructions but no calibrations, and asked to measure both the length and circumference of their partners' erect penis. Three months later they were asked to repeat these measures. Mean length on first measurement was 15.3 cm and 15.2 cm on second measurement. Mean girth at first measurement was 12.5 cm and 12.6 cm at second measurement. Test-retest reliability of measurement was found to be moderately low at r = .60 for length and r = .53 for girth. No relation was found between measurement discrepancy and the age, social class, education, ethnicity, or employment status of the partner taking the measurements. Although self-measurement strips are both convenient and acceptable, and widely reported in the literature, they only have moderate test-retest reliability. This may be due to both natural variability in penis size within subjects over time and unreliability of the measurement method.