The epidemiological part of the Huume tietokanta (HUUTI) consortium research project is the first large-scale longitudinal study of treatment-seeking illicit drug abusers in Finland. The objective of this report was to describe the sociodemographic characteristics and drug abuse patterns of treatment-seeking clients at their first visit. This study analysed baseline data of 4817 clients (3365 men and 1452 women) aged 11?65 years who sought treatment for drug abuse between 1997 and 2008 at Helsinki Deaconess Institute. Data were collected using a structured questionnaire. The majority (56%) of clients were between 15 and 24 years, educated at elementary school level (75%), and unemployed (57%). Opiates (30%) were the primary drugs of abuse. The primary drugs were mostly injected (45%) and were abused daily during the past month (44%). Cannabis was the most common secondary drug of abuse (34%). The secondary drugs were predominantly smoked (39%) or taken orally (38%) and were abused once per week or less frequently during the past month (33%). Age at initiation of illicit drug abuse ranged from 5 to 49 years. Polydrug abuse was common, with a mean consumption of 3.5 concurrent polydrug use, which were combined from 3 or more drug classes. The prevalence of lifetime/ever intravenous drug abuse was 64% and past month intravenous drug abuse was 64%, respectively, and 13% reported sharing injecting equipment during the past month. Early initiation, polydrug abuse, and risky consumption of illicit drugs were major areas of concern among the study population. Injecting drug use could place considerable burden on health services in view of complications and transmission of infectious diseases.