Nicotinic acid (NA) and nicotinamide (NAM), commonly called niacin, are the dietary precursors for NAD(+) (nicotinamide adenine dinucleotide), which is required for DNA synthesis, as well as for the activity of the enzyme poly(ADP-ribose) polymerase-1 (PARP-1; EC 2.4.2.30) for which NAD(+) is the sole substrate. The enzyme PARP-1 is highly activated by DNA strand breaks during the cellular genotoxic stress response, is involved in base excision repair, plays a role in p53 expression and activation, and hence, is thought to be important for genomic stability. In this review, first the absorption, metabolism of niacin to NAD(+), as well as the assessment of niacin status are discussed. Since NAD(+) is important for PARP-1 activity, various aspects of PARP-1 in relation to DNA synthesis and repair, and regulation of gene expression are addressed. This is followed by a discussion on interactions between dietary methyl donor deficiency, niacin status, PARP-1 activity and genomic stability. In vitro studies show that PARP-1 function is impaired and genomic stability decreased when cells are either depleted from NAD(+) or incubated with high concentrations of NAM which is a PARP-1 inhibitor. In vitro as well as animal studies indicate that niacin deficiency increases genomic instability especially in combination with genotoxic and oxidative stress. Niacin deficiency may also increase the risk for certain tumors. Preliminary data suggest that niacin supplementation may protect against UV-induced tumors of the skin in mice, but data on similar preventive effects in humans are not available. NAM has been shown in vitro to have an antioxidant activity comparable to that of ascorbic acid. Data on niacin status and genomic stability in vivo in humans are limited and yield ambiguous results. Therefore, no firm conclusions with respect to optimal niacin intake are possible. As a consequence of oral niacin supplementation, however, NAM levels in the body may increase, which may result in inhibition of PARP-1 and increased genomic instability. More studies are needed to define an optimal level of niacin nutriture in relation to genomic stability and tumorigenesis.