OBJECTIVES: Previous researchers (Ren, 1999) have used a 5-item shortened version of the Medical Outcomes Study Social Support Scale (MOS-SSS; Sherborne, 1991), but the measurement properties of this brief measure have not been established. We filled this gap by evaluating the psychometric properties of a 5-item perceived social support instrument (the SS-5) and testing equivalence between paper and web-based administration modes.  METHODS:  The SS-5 and other questionnaire measures were administered to a sample of adults recruited through newspaper and web-based advertisements in 8 US cities.  Participants were randomized to complete the SS-5 on either paper or computerized format followed by a one-week retest.  The intraclass correlation coefficient (ICC) was calculated to assess reproducibility of the SS-5 and equivalence between administration modes.  Cronbach’s alpha was calculated to assess internal consistency.  To assess known groups’ validity, differences in SS-5 scores between marital status/living arrangement groups and (separately), trichotomous groupings of a validated measure of dispositional optimism, the Life Orientation Test (LOT-R), were assessed with analysis of variance (ANOVA) models. RESULTS: Of the 258 participants that completed the baseline assessment, 251 (97%) completed the retest.  Mean age of participants was 48.6 years, 61% were female, and 71% were Caucasian.  Participants included 44% married or living with a partner; 24% had never been married. The mean SS-5 score was 17.3±4.9, and the ICC between paper and web-based was 0.89.  Reproducibility was strong (ICC=0.92), and the instrument was internally consistent (alpha=0.88).  The SS-5 significantly discriminated between participants living with a partner and those living alone (means: 19.1 vs. 15.9; p<0.001) and tertiles of the LOT-R.  The correlation of SS-5 scores to the full MOS-SSS was 0.91. CONCLUSIONS: The SS-5 was observed to have adequate reproducibility and internal consistency; and demonstrated appropriate known groups validity. Equivalence between paper and web-based administration was demonstrated.