OBJECTIVE: The purpose of this study was to assess the differences between early and late drop-outs and completers in the continuation phase of a clinical trial. METHODS: The authors studied 119 outpatients who were treatment responders in an 8-week open trial with fluoxetine 20 mg/day, and who were then enrolled in a 26-week clinical trial comparing the efficacy of fluoxetine versus fluoxetine and cognitive behavior therapy (CBT). Patients were assessed using the Structured Clinical Interview for DSM-III-R--Axis I (SCID-Patient Edition), Hamilton Depression Rating Scale (HAMD-17) and the following self-rated scales: Beck Depression Inventory (BDI), Beck Hopelessness Scale (BHS), Anxiety Sensitivity Index (ASI) and the Symptom Questionnaire (SQ) prior to starting the 26-week continuation phase. We defined "early drop-outs" (EDs) as patients who dropped out either at or prior to Visit 2 (which was at 2 months into the 6-month continuation phase); those dropping out at Visit 3 or later were defined as "late drop-outs" (LDs) (ED < or = 2 months; LD >2 months). The Kruskal-Wallis and the Mann-Whitney U tests were used for data analysis. RESULTS: Of the 119 patients, 83 were completers (mean age: 42.1+/-9.0 years; 46 [55%] women; age of onset of major depressive disorder [MDD] = 24.3+/-12.5 years), II were EDs (mean age: 38.1 + 13.0 years: 4 [36%] women; age of onset of MDD = 22.0+/-11.1 years) and 25 were LDs (mean age: 35.2+/-10.4 years; 12 [48%] women; age of onset of MDD = 24.6+/-11.6 years). LDs were significantly younger than completers (P<.01). There was no significant difference in age between EDs and LDs, nor between EDs and completers. EDs were more likely to have been depressed for a longer period of time compared to LDs (P< .05). EDs completers were depressed for a longer period of time compared to LDs (P< .05). CONCLUSIONS: Our data suggest that late drop-outs are significantly younger than completers, although age is not a predictor between early drop-outs and late drop-outs. Further, early drop-outs are depressed for a longer duration compared to late drop-outs completers are depressed for a longer duration than late dropouts, and Early drop-outs have significantly more social impairment compared to completers. Our study identified some patient characteristics significantly associated with dropping out of a long-term clinical trial.