This study explores relationships between Taiwanese Sign Language (TSL) and Chinese reading comprehension with 65 deaf adults/youth (study 1: ages 14--50 years) and 27 deaf children (study 2: ages 6--13 years). Significant relationships were found between scores on two tests measuring TSL tasks. In study #1, the TSL tasks (measured by Taiwan Sign Language Comprehension Test, TSLCT) on story and syntax comprehension predicted more of the variance than vocabulary on Chinese reading comprehension. In study #2, TSL tasks measuring vocabulary (using Taiwanese Sign Language Receptive Skills Test, TSL-RST) predicted more of the variance than syntax and story comprehension on Chinese reading comprehension but were not significant. Results are interpreted utilising Cummins' Linguistic Interdependence Theory and Threshold Hypothesis. Limitations, future research and educational implications are provided.