This article analyzes the test-retest reliability of network size and density measures originally implemented in face-to-face surveys, but implemented in this research in a telephone survey for examining “Social Relationships among Czech Citizens.” For this purpose we can divide family and friendship networks. Network size is measured with the number of family members; respondent’s friends at work, in the neighbourhood, as well as other friends. Network density is operationalised as frequency of contact with family members and friends. The analyses show a high test-retest reliability of the network size and density measures of the family network, but very low reliability for the measures of the friendship network. More detailed analysis reveals that the low reliability of network size, density of friends at work, and contact frequency with friends, can be explained in terms of respondent characteristics. In contrast, the low level of reliability exhibited by other variables is independent of respondent characteristics., Julia Häuberer., and Obsahuje bibliografii a bibliografické odkazy