Prior research at Crossroads Centre Antigua has shown a 15% increase in long-term abstinence rates of clients if a family member attended the family program. This study examines the association of family program participation on client residential treatment completion. A retrospective review of records of all clients admitted from January 2004 to June 2007 was performed. Participation of client family members in the family program (a 4-day program including didactic material, workshops, group and individual counseling sessions for interested family members) was recorded. Information obtained from a limited client database, in addition to family program participation, included age, gender, country of residence, drug of choice, and 29-day treatment program completion. Data from clients having family members participate in the family program were compared with clients not having family members participate. There were 1,014 clients admitted, 681/1,014 (67%) men, mean age 41years, 733/1,014 (72%) from the United States. There were 187/1,014 (18%) clients using opioids either alone or with other drugs, 330/1,014 (33%) using cocaine or another stimulant drug alone or with other drugs, and 497/1,014 (49%) using alcohol or another non-prescribed sedative drug alone or with other drugs not including opioids or cocaine/stimulants. Of these clients, 207/1,014 (27%) had 416 family members (155/416 [37%] parents or step-parents; 133/416 [32%] spouses, significant others, or partners; 67/416 [16%] siblings or step-siblings; 38/416 [9%] children or step-children; and 23/416 [6%] other individuals) attend the family program. For all clients admitted, 902/1,014 (89%) completed the 29-day residential stay. The clients who had family members participate in the family program were significantly more likely to complete their stay (266/270 [99%]) than those clients who did not have family members participate (636/744 [86%]; chi-square p < .0005. Those with family members attending had no significant difference in other factors that may affect treatment completion, such as age (40 vs. 41 years), gender (65% vs. 68% male), country of residence (73% vs. 71% from the United States), or opioid drug (17% vs. 19%), cocaine/stimulant (34% vs. 32%) or alcohol/sedative as primary drug (49% vs. 49%). These data suggest that participation of a client family member in a family program, independent of other clinical risk factors, is associated with significantly improved treatment retention, a surrogate marker for better long-term abstinence rates following residential treatment.