The purpose of this research is to measure cell phone use among high school adolescents and the factors associated with intensive cell phone use (depressive symptoms, social isolation, drug and alcohol use, school failure, and cell phone dependence). We conducted a cross-sectional survey study of 1,328 adolescents aged 13 to 20 years in nine secondary schools of the Community of Madrid between January to April 2007. The mean age of sample participants was 15.7 years. Almost all (96.5%) had their own cell phone (80.5% had one, and 15.9% had two or more). Some 54.8% take it to school and 46.1% keep it on during class; 41.7% use it intensively. The estimated prevalence of cell phone dependence was 20% (26.1% in females, 13% in males). Intensive cell phone use was associated with female sex, rural school location, good family economy, smoking tobacco, excessive alcohol consumption, depression, cell phone dependence, and school failure. More health education is needed to promote correct and effective cell phone use among adolescents. Factors associated with intensive use and dependence should be considered for possible intervention activities.