Reduced responsiveness to joint attention (RJA), as assessed by the Early Social Communication Scales (ESCS), is predictive of both subsequent language difficulties and autism diagnosis. Eye-tracking measurement of RJA is a promising prognostic tool because it is highly precise and standardized. However, the construct validity of eye-tracking assessments of RJA has not been established. By comparing RJA an eye-tracking paradigm to responsiveness to joint attention during the ESCS, the current study evaluated the construct validity of an eye-tracking assessment of RJA for 18-month-old infant siblings of children with autism. Relations between measures of RJA and concurrent language skills and autistic symptomatology were assessed. Correlations between measures of ESCS RJA and eye-tracking RJA were statistically significant, but few relations between either ESCS or eye-tracking assessments of RJA and language or symptoms were observed. This study establishes the construct validity of eye-tracking assessments of RJA.