the relative merits of in-patient and out-patient treatment of substance abuse have been widely debated. for severe, chronic clients, the best form of treatment may be intensive in-patient care. less severe clients may fare better with out-patient treatment. regardless of the type of treatment, clients' drop-out rates are high. since the client may be three times as likely to be free from drugs one year later if they complete treatment, serious attempts need to made to determine the factors affecting client drop-out. the research examined this issue by means of an archival search of client records from the lakehead addiction centre treatment program at the lakehead psychiatric hospital (lph) in thunder bay, ontario. the demographic, personality, and social stability characteristics related to drop-out of clients who had attended either the in-patient or out-patient program were examined. treatment drop-outs were studied for 98 out-patients and 406 in-patients. this study confirms research which found a high rate of early attrition from treatment for substance-abusing clients. the results indicate that treatment completers in either program differed significantly from non-completers by: patient type (p<0.05), use of lsd (p<0.01), and treatment mandated (p<0.05). out-patients had significantly more completers. this may be due to the significant differences between in-patient and out-patient attenders. these differences included: social support (p<0.01), attendance at aa/na (p<0.01), and maximum drug intake per day or binge (p<0.05). natives were found to be significantly more likely to drop-out of either treatment (p<0.01).