The aims of this study were: (1) to examine the prevalence of symptoms of problematic cellular phone use (CPU); (2) to examine the associations between the symptoms of problematic CPU, functional impairment caused by CPU and the characteristics of CPU; (3) to establish the optimal cut-off point of the number of symptoms for functional impairment caused by CPU; and (4) to examine the association between problematic CPU and depression in adolescents. A total of 10,191 adolescent students in Southern Taiwan were recruited into this study. Participants' self-reported symptoms of problematic CPU and functional impairments caused by CPU were collected. The associations of symptoms of problematic CPU with functional impairments and with the characteristics of CPU were examined. The cut-off point of the number of symptoms for functional impairment was also determined. The association between problematic CPU and depression was examined by logistic regression analysis. The results indicated that the symptoms of problematic CPU were prevalent in adolescents. The adolescents who had any one of the symptoms of problematic CPU were more likely to report at least one dimension of functional impairment caused by CPU, called more on cellular phones, sent more text messages, or spent more time and higher fees on CPU. Having four or more symptoms of problematic CPU had the highest potential to differentiate between the adolescents with and without functional impairment caused by CPU. Adolescents who had significant depression were more likely to have four or more symptoms of problematic CPU. The results of this study may provide a basis for detecting symptoms of problematic CPU in adolescents. (Contains 4 tables.)