SETTING: After the diagnosis of a case of tuberculosis (TB), contact tracing is directed by the risk of transmission, for which sputum acid-fast bacilli (AFB) staining results are highly relevant. Limited data are available on the effect of the degree of acid-fast positivity, of a polymerase chain reaction (PCR) result or of bronchoalveolar lavage (BAL) fluid results on the risk of transmission. OBJECTIVES: To investigate factors associated with TB transmission, focusing on quantitative sputum smear results. DESIGN: Retrospective study of contact investigations performed over a period of 5 years in a Dutch Municipal Health Service among all index patients with TB, and the tuberculin skin test and chest radiography results in contacts. Three definitions of transmission were used: >/= 1 or >/= 5 contacts with positive TST or active TB in contacts. RESULTS: The highest (+4/+5) sputum AFB grades were associated with the highest relative risk (>/= 8) of extensive transmission or active TB among contacts. Novel risk factors observed were employment or school attendance, positive PCR of sputum and positive AFB staining of BAL fluid. Pulmonary symptoms, infiltrate or cavity and positive AFB sputum stain were also associated with transmission, confirming previous studies. CONCLUSION: The risk factors observed in this study may aid in the extension of contact investigations.