OBJECTIVES: This study examined weekly patterns of drug treatment attendance in relation to date of welfare payment receipt and reason for treatment absence. METHODS: Treatment attendance by Medicaid-eligible pregnant women who were drug dependent was examined by calendar week over a 29-month period. RESULTS: Time series analyses showed that attendance was lower during week 1 than week 4. Drug use was the most frequently reported reason for treatment absence during week 1 (25%) but was not reported as a reason during week 3. CONCLUSIONS: Drug-dependent outpatients had increased absences associated with illicit drug use during the first week of the month when welfare payments were received. The generalizability of the findings is unknown.